Anytime the top is 'parked' in either the "up" or "down" position, all pressure is released. I do not believe there is a discernable difference in level once the system pressure is released.

So, if it is low, it is low. And if it is low, you have a leak. Now, I say that, but in the Three roadsters I've owned, I've never checked the level beyond peering into the cavity down there and noticing the level was something less than full - maybe some are shipped low, I do'nt know.

If the top is giving you trouble and the level is low - I'd say you have a problem here beyond just adding fluid. But you have not explained what the trouble(s) is, so I perhaps should not comment.