Topping off an S90 system is virtually impossible, only because there isn't any way that I know of, to do it while reading the high side pressure (no port).

The S90 is best sucked out, then recharged by weight. That works well.

If you are going to 'top off', do so carefully. The low side pressure tells you only the output of the evaporator, not much else. The high pressure reading is where the best information is.

Part of the reason that the system won't suck out the can of refrigerant, could be because there was too much low-side pressure. A simple way to get around that, is to charge with the doors open and a garden hose in the condenser. But you can easily overcharge that way too. And a slight overcharge on an R134 system is worse than a slight undercharge.