What shock’s on it? I did a similar thing with my Cube Stereo 140 with a 200×57 X Fusion shock because I wanted a strongish short travel FS with ISCG tabs, and the frames were cheap new.
Basically, and this is very basic and assumes that it actually has 140mm of travel (in the case of my frame) and not 135 or 145, the amount of travel divided by the shock stroke will give you (roughly) the ratio of rear wheel travel to shock travel, so in the case of the Cube it’s 2.46:1. I wanted around 120mm which meant reducing the shock stroke down to around 48mm to get it roughly there or thereabouts.
Shock out, air can off, clip a 5mm nylon spacer around the 10mm piston shaft to reduce the travel on compression and a 2mm O-ring (I just used the travel indicator ring that came on the shock) on the other end that the air can slides over (is this called a stanchion?) to stop it topping out as far and the overall effect is a reduction in rear wheel travel to around 125mm and a slightly lower BB and slacker head angle (because the BB’s dropped).
I’ve tried just putting more air in it and reducing the compression travel and it didn’t really work, it needed to have the overall length reduced slightly too to make the bike sit lower, unsagged.
I’ve probably made it sound way more complicated that it actually is, if the shock’s basic enough it takes about an hour which includes making a cuppa and it’s really easy to go back to standard if you want to.