The attached notebook illustrates the "Ernst angle", which is the flip angle that maximizes steady state transverse magnetization for a given recovery delay between successive scans in an NMR experiment. (Output has been stripped due to strict file upload size on this forum)
The Ernst angle is close to 90 degrees if the delay between scans allows near-complete longitudinal recovery, i.e. is a few times T1.
For a recovery delay that is short compared to T1, the Ernst angle is substantially less than 90 degrees.