technically digital doesn't have _any_ headroom. anything over 0 at the top and its crap. to compensate engineers pick a value, usually between -20dbFS to -12dBFS. tape has a _loose_ headroom of 14dB, add a few more for errors just to be sure... -12dbFS is on the risky side, i'd try aiming for averaging -18 or so.
is the recorder 16bit or 24bit? when mixing down to a 16bit format (DAT or whatever) one would do a few practice run throughs of the song, trying to get the peak as close to the top without overs. since playing the mix again and again is entirely repeatable, this works. when tracking someone live, you obviously down know whats coming next, so aim for you average level to be safe, leaving headroom just in case.
if mixing down to a 24bit format, just make sure it averages somewhere above -20dB and don't worry about it. there is plenty of resolution so you don't need to nit pick.
again, 16bit is another thing, aim for getting it a few dB under 0 by running through it a few times.
one more thing, be concious that there are several different ways to use the term "-12dB". on analog 0 was supposed to be you average, so there are values like +6dB and such. on digital there is nothing over 0 so analog's 0VU = -18dBFS or whatever.
if you are mixing entirely on the computer, mix to a 24bit file and just let it be, in mastering (whoever that may be) the final peak adjustments can be made.