Okay here is what you do:
320nWb/m is 5db over 185nWb/m. 185nWb/m is referenced to 0db in the world of tape engineering. This means that you are going to be shifting your VU meters -5db referenced to 0db. What does that mean to you....?
You are going to be using tapes such as 456 or GP9 or one of the newer tapes that has come out. Each of these has a different amount of fluxivity or magnetism headroom that they can maintain with out distorting the waveform. In the case of 456, it is a +6db over 0vu type of tape (referenced to 185 nWb/m). This means that if your calibration tape were 185 nWb/m (not the 320 nW/m tape you have) and you were calibrating to use 456 you would set your repro electronics at 1k, 10k, 100hz to read -6vu while doing the repro alignment. You would then set your record electronics to read 0vu after the repro electronics had been set during the record alignment.
So what does this mean to you...?
With your MRL tape you are going to cal your machine like this:
1) Demag the tape path. (Make sure you know how to do this properly and have a powerful enough tool for the task. Han-D-Mag is the guy to get. Also, if you do this wrong you can actually magnetize the heads. So know what your doing.)
2) Clean the tape path with 99.9% isopropyl alcohol. Do NOT clean the rubber components with this. If at some point you do need to do clean the rubber use Fantastic Orange Action. This was recommended to me by the tape machine techs at Sony Studios here in NY. I think it works great too and won't eat up or dry out the rubber.
3) Put the MRL tape on the machine. Make sure the machine is not record enabled.
4) Play the 1k tone off the MRL tape. With your 320nWb/m tape you are going to set the "level" of your repro potentiometer to read -1vu for a +6 tape and -4vu for a +9 tape. 456 is a +6 tape. GP 9 is a +9 tape. 499 is also a +9 tape.
5) After you set your 1k tone, do the same thing for 10k and then the same thing for 100hz. The 100hz however will get set properly during the record alignment and you are only ball parking it now.
6) After you have finished the repro. Do the record cal.
7) Take the reel of tape you are going to record on. If its a new tape, put the reel on the right hand side otherwise know as the "take-up side" upside down. Rewind the tape onto the left side, called the "supply side". You do this so that the tape is stored "tails-out" and this will diminish the effects of "print through".
Splice leader tape on the front of the record reel.
9) The first thing to set is over bias and now that your repro electronics have been adjusted to give reference for your record levels, send 10k to your machine with the machine set to input mode. Set the meters for 0vu by adjusting the send from your console/DAW. Then switch the machine to repro and hit record. Turn the treble potentiometer counter-clock-wise til the needle falls to -80db or until you have no level. Start turning the Treble pot clockwise until you see the needle peak. At that point, continue turning the Treble pot clockwise while the needle starts to "fall backwards" towards the quiet side of the meter counting db until you are at the correct number of db of "over-bias".
9b) Every type of machine has different electronics and head construction. I am going to tell you how to set bias on an ATR-100. If you have something different this may not apply to you. Setting bias is directly dependent on tape type and speed. Limiting the conversation to 456 and GP9, 15ips and 30ips you will over-bias like this: At 15ips you will over-bias aprox. 4db with 499 and GP9; 3db with 456. At 20ips you will over-bias approx. 1.75 db with 499 and GP9; 1.5 db with 456.
9c) My machine has slightly different heads from stock so its a tad different from stock ATR settings. Also, over-bias is not exact. You will get different sonic effects from varying the over-bias. Someone went into that in detail a while back on these forums I think. Do a search and you will find it. A brief description of over-bias: It is high frequency magnetism that puts the magnetic particles in a state of motion as they pass over the record head so that they will record the audio portion in greater detail. This is a crude explanation, very lay, but should help you visualize whats going on. In the case of setting large over-bias settings, it might be useful to you to send the machine less than 0vu of the 10k tone so that you have enough room on the vu meter to watch and accurately calculate see where the needle peaks.
10) After you set over-bias, set the record level. Send 1k to the machine with the machine set to input mode. Set the meters for 0vu by adjusting the send from your console/DAW. Switch the machine over to repro, hit record and set the "level" potentiometer on the record card/electronics to 0vu. Do this for 10k as well, adjusting the Treble pot.
11) Then send 100hz to the machine with the machine set to input mode. Set the meters for 0vu by adjusting the send from your console/DAW. Switch the machine over to repro, hit record and adjust the repro electronics Bass potentiometer for 0vu. You will notice that there is no adjustment for Low Frequency on the record electronics so this is how you set the low end on your machine.
IMPORTANT: All of this is predicated on a machine that has had proper I/O and VU meter calibrations and proper azimuth adjusments performed. Its been made clear to me that really the best way to feel good about your azimuth is to use a Oscope set to xy and to get your outside channels to make as close a straight line on the diagonal from 7:30 to 2:30 if you were to visualize a straight line connecting the dots on a clock face.
Good luck with it,
josh