My missus has a RS Sid RL but isn't able to get much performance from it due to being a real lightweight at less than 50kg. To get anywhere near the 25% sag she can only put about 45 psi in and then she'll only use about 50% travel in use. I know we could change the oil weight for damping purposes but is it possible to tune the air spring to alow for lightweight rider's ?
If it's only air in the fork (no coil) then yes, keep dropping the pressure until you get the required performance. I run 45psi in my Lyriks (I'm 58kg and ride quite aggressively) so I guess she'd need it in the 30's for 30% sag and decent performance.
What he said, basically.
Check the max. travel used with an O ring or a cable tie (not too tight) to help you get to the right pressure. Lower the pressure until she bottoms out on most typical rides.