I don't think you're supposed to "measure" the amount.
You could get the exact specs, but most of the peds I've seen have a drain plug and a fill plug.
you remove both, drain all the fluid out (by tipping your bike to the side...)
then, with the bike totally level, you fill through the top hole until the first little drops come out of the bottom hole.
If your bike is "level" nothing should be dripping... but if you tip it even a little bit, you should see that fluid...ready to roll out.
nearly impossible to 'overfill' because the excess will overflow right out of the bottom hole.
regulars: correct me if I'm off target.