So...this thread makes a bit more sense now....
The bottom line is: You won't know until you chrony the rifle with 0.20g BBs.
But...
1. Manufactures often report the highest FPS that they read off a test model...and the hopup may or may not be on. (without it on, you'll get the highest number...good for the marketing guys). They may also chrony with lighter than 0.20g BBs...so that 490fps rifle might not really be what's printed on the box so far as the local fields are concerned.
2. Springs/setups...IMHO...settle in a bit most times. Dropping a couple of FPS (unknown how many) is not uncommon. Who knows how long that mechbox has been assembled for and what state (piston forward vs. other) it's in.
3. As a general rule guys chrony their guns with their hopups set to where they're going to use it. And...turning on the hopup a bit does drop the FPS of the BB a bit...sometimes dramatically.
So...again see above...you won't know until it's chronied. Get out to a game field...set the hopup. Then chrony it. If it's hot...at least you'll know by how much (it'll give you a hint as to which spring to buy, etc...).
Aim lower than the maximum...if it's 350 aim for 300-330. If it's 400 aim for 360-390. The last 10fps doesn't matter at all...and not every setup is absolutely predictable. I.e. swapping a M100 from a M120 in my rifle might end up dropping the FPS from 420 to 340...but doing the same in yours might yield very different results. Go back to the chrony to be sure.
Being new, not knowing your way around a mechbox and having to swap out a spring is a pain in the ass. Get someone else to do it for you
Best of luck,
Tys