Thanks for your reply. I have already calculated 31.49px/mm previously through several different calculations and there are many calculations that result in 31.49px/mm. However, I believe this sensitivity is incorrect as ingame it didnt feel right and felt too slow. I wanted to know objectively or if it was just bias. So, ingame I went up to a wall that has straight vertical lines. I got my mouse and attempted to move my mouse exactly 216mm horizontally and marked where the crosshair ended up. Then, I got my pen and moved 216mm horizontally. Moving the pen horizontally exactly 216mm is always consistent. I adjusted the px/mm sensitivity in OTD, and eventually reached approximately 35px/mm to nearly match up with the mouse. In OTD, using 31.49px/mm was clear that it was too slow as it never matched up when moving my pen 216mm horizontally ingame compared to when I moved my mouse 216mm.
The problem is that moving the mouse 216mm is always inconsistent, so I do not know the exact measurement in px/mm. I used websites that measure DPI, and found that 31.49px/mm gives approximately ~650dpi when moving 216mm, and ~35px/mm gives approximately 750-820dpi. This also helped me to conclude 31.49px/mm is an incorrect sensitivity.
I found a program that measures the number of pixels moved: mouse monitor this program is very useful as it tells me how many pixels it takes to move my pen 216mm horizontally. For 35px/mm, it will move a consistent 7560pixels. That is because 35*216=7560. Now, I tried this on my mouse. Several attempts trying to move my mouse 216mm resulted in approximately ~7500pixels, but it was always inconsistent. I need to know a consistent result of how many pixels my mouse will move if moved 216mm horizontally. Knowing this unknown result means that I can easily calculate the px/mm for OTD. Apologies for the repitition, I just want to be precise with my wording.