Do you plan to record from LPs often? If so, and if the Onkyo settings change over time (they shouldn't), maybe a resonably-priced outboard phono preamp would be easier. I don't mean to spend your money for you, but it may be esaier and faster in the long run.
If you use Zone 2 output for this, make sure the recording input level is fairly high- if it's too low, you lose the ability to adjust in fine increments and it's more likely to cause distortion at the computer's input- Rec Out ISN'T the same as the pre-outs because recorders want about 1V.
I would refer to Pg 153 for Zone 2 setup, then set the maximum output to about 60 (if you use the 0-100 scale) with the computer's recording input set to about 7/10 if it has a slider record level control, or 1:00 if it has a rotary control. The level should never go above +3VU for anything but the strongest peaks and really, I wouldn't go higher than +2VU if you want the recording to be as clean as possible, assuming the LPs have decent dynamic range. Before CDs, many Pop & Rock LPs had about 20dB of dynamic range, classical may have 30dB if the recording was particularly good. Remember, this is usable dynamics, not 'level of hiss to max".
Use Zone 2 Volume control to find the average maximum level, then set it to the closest point that corresponds to one of the 10s (-20, -30, -40, etc) if you use the 1-100 scale. I use -dB whenever I set up an AVR because it's associated with audio level far more than 1-100.
