The sad truth about that spring in the box...and the one in the gun is that you'll never know what they'll shoot until you install it and shoot it through a chrony.
It seems a bit of an investment...but buying a chrony pays off in the long run...and if you decide to sell it, well...it's pretty obvious that there's always someone out there looking for one, right?
The doubly sad part is that buying a M90/100/110/etc and installing one of them won't really make sure you're shooting under this or that limit. It's a pretty good GUESS...but it still sucks to show up at a game and not be allowed to play with a hot gun.
I had a knock about CYMA AKS74U (itty bitty AK) that shot 480fps with a M110 spring. I finally got it DOWN to just under 430 with a M100 spring!! (with a M120 it sounded like it'd punch right through your gear...LOL)
So...NEVER simply say "I'll toss this part in and that part in and I'll end up with this". Get a chrony and know for sure (or within an acceptable range of error).
Personally...I'd rather use a chrony vs fiddling with mics and timing sound charts. It may not be as accurate...but it's quick, repeatable and easy.