Suggested FPS for 25 cal

Recently purchased a Umarex Gauntlet 2 25 cal. Which, I am really enjoying. However, not sure if I am getting the fps correct. Currently do not have a chronograph. Everything is stock, the regulator is 2100, and using JSB Match Diabolo Exact King MKII Heavy .25 Cal. Which have been working well. Wondering am I going to light or heavy. Live in the Midwest and getting cold going into winter.
 
I have one of the clamp on and I agree it is worth having. I use my tripod mount almost all the time now, however. It is third from the left on the aliexpress link above under the clamp on chrony. The clamp on shifts my point of impact when I clamp it onto my guns. The tripod does not. So I can shoot targets and measure velocity at the same time. The only tricky thing is you have to make sure the gun is reasonably centered in the shooting window of the chrony. I've messed up several times, however, and shot one of the posts. The chrony has survived so far. The clamp on would not have that problem, it's pretty hard to mess setting it up. Both have rechargable batteries that last a long time. They are suprisingly nice considering the price. I can't imagine spending what FX wants for theirs when these work so well.