I always dialed in my scope on my M24 at 30ft in my basement using no lighter than 0.30g BBs, and just tweaked it for distance and hop up setting at the field if needed. If you have an indoor range of 100ft, even better.
And don't bother using anything lighter than 0.30g, you won't get any type of performance you were hoping for out of a sniper rifle.
As for velocity, I used to run at 500fps w/0.20g BBs, and use heavy ammo, would get quite a few flyers that would take off on their own within 120ft or so, later found out that BBs are unstable at high velocity. Past few years I played I had the 24 set at 400fps (w/0.20g)and the performance is better than any I've seen when using 0.36g BBs. Nutshell, don't get fooled into thinking faster BBs give you more range, they don't, they just create a more unstable environment for a little ball to deal with when travelling through unstable air conditions.