You might want to look into a "linear slide rail" set-up. They are relatively inexpensive and have mounts for your feet and bolt holes in the sliding member to mount your moving rest. That would improve your repeatability. The spring is another area for variation... and also misleading information. A compression spring will skew your results on a curve. since you're looking for real world improvemnt created by your break you want to see actaul difference. A compression spring produces more force the more you compress it. You need a constant force spring... or a different way to keep the unit toned down for measurement within a reasonable distance.
Myself... I'd think simply adding weight to the moving member and eliminating the spring would give you a pretty darn accurate comparison of break vs. no break and not introduce the spring variable.
Slide rail with plate on top of slides that you mount your rest on... Hanging below plate on both sides are square tubes full of lead? their low center of gravity to the slide will also help counter the weight of the gun on top... sort of keeping your load centered to the slide bearing centerline. Play with the amount of weight depending on the caliber. A 22 wouldn't need any weight (no need for a muzzle break either...) where a 50 cal. would need a good bit.