what LEVEL to bring analog stereo tape into DAW for mastering purposes?

Recording tape playback to DAW? This I did ages ago, so long ago my memory is failing lol. I be concerned with headroom compared to recordings instruments straight to the DAW because excessive use of dynamic effects (maximizer) of 'Tape Recordings' will bring up the noise-floor i.e. tape hiss will be more noticeable particularly during the quieter parts and start/end of the song. Therefore, the level will need to be about -2 (DAW) pre-mastering, and with mastering use minimal processing (as explained)!   There is no straight up answer to exactly what level due to various reasons i.e. quality of tape machine and content captured. But of course there are noise reduction tools, I prefer to get is as best as possible (at the source)!.

IIRC: Recording at high levels to tape actually minimizes noticeable hiss... levels recorded too low? Tape hiss will be noticeable, especially so just prior to start/end and during the quieter parts of the song.  That said, I prefer to record on tape fairly hot but just below the distortion/clipping, all instrument parts are carefully checked by isolating/soloing the track to see if the highest possible peak levels are fine, and I also listen for the lowest possible level to decide if comp/limiting (and probably the use of noise gate especially i.e. dist/Guitars),therefore, it's possible that pre-post processing will be required before I proceed.... Not all tape machines are equal i.e. some tape units (reel to reel) levels are +6 or +9, and of course all machines require regular maintenance. Hmmm, I guess that's why I eventually moved entirely to ITB (DAW) lol! However, guitars recorded on tape is cool, an effect in and of itself which is really nice. I love 2" tape (to heck with 1/4") but for what I do, not worth the costs/maintenance of using tape.

There is no standard definition as to what signal level starts to clip the input of an ADC (it's device-dependent).  So, as hot as you can get it without clipping would probably give you the best S/N ratio and dynamic range to play with.  Then you can simply drop the level in the DAW to give yourself some headroom for mastering purposes.

ston
So, as hot as you can get it without clipping would probably give you the best S/N ratio and dynamic range to play with.

In the case of tape, it has so much less dynamic range and more noise that it really doesn't matter as long as you aren't clipping. IOW, the source is limiting the dynamic range and S/N achievable - and not by a little.