I re-leveled my 9500I the other day and apparently it was off by just a bit (didn’t look like it at all). I did this because of the configuration of the Horn, and to mount it stealthier.
I actually got an increase in detection range. My RD has a nice flat bottom to put a level on.
Your RD should be mounted striate, level and as centered as possible. Mounting high seems to be the consensus but mounting low may have advantages as well.
Leveling the horizontal and vertical plane requires only a small bubble level, vehicle sitting level and a flat surface on the RD. if possible knowing the horn configuration internally can help (did with my 9500I).
I installed my 9500I high with the top 1/2 under the tint and the bottom 1/2 just below the tint. The 9500's horn is mostly at the bottom half of the detector.
9500I horn: 