That puts bending stress close to 1000psi- too much for no.2's but okay for Select Structural. Thats not checking deflection though which I would guess would be high.
Space them at 12" o.c. and your bending stress is under 800 psi so would be okay for No 2's but still deflection may control.
VTEIT-
Just out of curiosity where did you come up with 800psi for 12" spacing? I just did a quick check and got over 1400 psi (and that's for no load other than the 75psf).
M=75*20*20/8*12=45000lb-in.
S=1.5*11.25*11.25/6=31.64in^3
fb=M/S=45000/31.64=1422psi
Again, that's neglecting any other load besides the 75 psf, which he's likely considering as a LL.
OP-
This tells me that the 16" spacing won't work, the 12" spacing probably won't work (depending on the grade) for strength and/or deflection.
Sorry I am answering a question with a question, but are these joists on a stamped drawing? Perhaps it's worth calling the engineer with a friendly inquiry.
