Hey,
In my opinion and experience, only 90mm would not be enough.
Even if you set your shock position so that you have enough bump stroke (which it sounds like you are doing with putting the spacer at the top of the tube) you will not have enough rebound stroke. you will top out a lot and it feels very harsh. The more you 'preload' the springs the harsher the clunk when you top out also.
Ideally you want more rebound stroke than bump stroke. This is what OEM runs and you still get topping more often than bottoming.
It's better to get the strut cut down to suit the insert, running the st185 cartridge I think you need about a 20~40mm spacer in the bottom of the tube and then get the tube cut & welded to match.