you could also buy a drop-in V3 gearbox with a less powerful spring inside which you can simply replace when you need a gun firing 350FPS or less (a gearbox with a M90 or M100 will do the trick - depending on the efficiency of the piston compression) - This way, you can switch between indoor and outdoor setting relatively quickly and easily (replacing a V3 gearbox in an AK takes about 15-20 minutes; while, opening the spring inside the gearbox yourself requires a lot more work and pratice unless you know your way around a mechbox
)
You also have to remember that Chrono readings are affected by a lot of factors (temperature for instance). Most field accept a +/- 5-10 margins on readings (in Quebec as far as I am concerned).