Also to add on to this,
According to the SPR chart, with effective stack sizes of 100BB. Playing heads up against 1 player, so raising to 3BB or 4BB, or even 5BB. Yields a SPR of {13.9, 11.3, 9.0 } And if your raising with a TPTK hitting kind of hand like AK, AQ, QK, thats the worse range of SPR's you should play at.

So not only are the fixed raise sizes and SPR concepts different, they are exactly opposite when working with an effective stack around 100BB.