Re: 2 different questions 1) Powder and 2) Spotting scope magnification
We go as high as 2 to 5 more grs of powder as the temp. drops into the freezing zone. This depends on the case capacity and bore diameter.
As per the 22X bigeyes, yes you can see the vapor trail with the 22X quite easy and at ranges out past 2000 yds. You can also see bullet impacts at that range into the ground or off of rocks VERY easily.
Regular binoculars (even expensive ones) don't always give good definition at extended range because of the shorter focal length and smaller eyepiece diameter. The bigeyes are the way to go as per much less eye strain and definition all the way to the target when watching the vapor trail.
22X wide angle is the most used power in the bigeyes especially in the 60 MM Spacemasters.
You can really see the 338 300 gr vapor trail at 3300 FPS, even 15 yards to the side of the shooter instead of directly behind him.