I really depends on barrel length you will loose velocity fast going down from 6" barrel. I usually think in terms of supersonic- subsonic, and it seems pretty much all "normal" hi-velocity stuff goes subsonic when fired from 5-6" barrel. So I guess 150fps is about right, I thoguht it is closer to 250fps loss of speed. I can see why stingers loose much more - more powder burns outside the barrel.
Regarding distances. There are distinctively two different disciplines - bulleye target shooting and action shooting. Bulleye is better off at 25m as this is olimpic distance. But if never had any formal caching I may suggest you move a little closer or it may be hard to see where did all holes go. And some kind of confidence builder.
For action shooting, whatever distance you can hit silhouette target from. When I start something new I usually move up to 7 yards, sometimes as close as 4-5 yards but I recommend not to dwell on success at short ranges, move up earlier and speed up your learning process. Do some reading, that will save ou a lot of frustration and ammo on learning simple things everybody already know.