With it full if gas, fully compress it and allow it to extend. You can fiddle with the rebound adjuster at this point to check that it's functioning properly.

Also, if it doesn't fully extend then there's air in there. Or if when you fully compress it it quickly moves a couple of mm before the rebound damper takes effect, same problem.

Anyway, good on ya for doing it.

FYI, the shimming on the stock shock and the F3 shimming are very different. Early F3 shocks also have a very different valve. Later F3 shocks use the same valve as an NSR, which is same as CBR250RR, VFR400, RVF400 and plenty more I'm sure. In fact, they all mostly use bits from the same parts bins.

I've not tried a double shim stack on the rear, but it works on the forks. Gives a real plush ride around the town when you're cruising, but then stiffens up at full tilt.