Installing a non linear spring backwards should give you a drop in FPS but your gun would still be a bit hot, maybe you can mess with other settings/parts to drop it to 300.
Also that guy who sold you the spring either didn't know (wrong information) or purposely sold you the wrong spring. a PDI 120 will give you 327 FPS according to the ASC spring chart (which a lot of other sites use BTW, such as Redwolf).
A stock TM spring like someone else said will bring your gun down to below 300 (90 M/s = ~290 FPS).
Also keeping the spring compressed for a long period of time should break it in enough that it shoots lower although I don't think that's recommended so only use that as a last resort if you're only a few FPS above the limit.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE