Although I agree that milling a very small RDS like the Viper into the slide will achieve the lowest red dot; I also understand the desire to NOT mill the slide, have a very reliable set-up with BUIS, and keep the total investment reasonable.
So here's a solution that's worked so well on my full-size 75 SP-01,
that I'm just now doing to my P-01 too:
The combination achieves lower 1/3 co-witness BUIS with only a .275 tall front Dawson FO sight, a self-adjusting J-Point RDS that stays ready 24/7 but dims in storage for 1 year+ battery life, and a Leupold #170903 CZ-75 mount that utilizes the dovetail to mount low and contours perfectly to the slide. ALL-IN only $325. The J-point fits Leupold mount exactly (made by very experienced RDS company Shield for JP Enterprises) has a built in notch that serves as a rear 'iron'. I posted a detailed DIY for CZ 75 Red Dot <$325 in 'Lights, Lasers, Red Dots' Forum section. I'll post more images of my P-01 as I complete my P-01 later this week.
Of course in use the red dot will be in center of reticle, but image below shows co-witness of 'irons' and red dot. JPoint is only 3/8" above top of slide. Of course, there's no reason you can't mill the J-Point to achieve an even lower mount, but this set-up is very good!
I won't argue that a deep-milled Trijicon RMR or Viper without an adapter plate might be an even better and perhaps more rugged set-up, but you're talking more $$ invested; and cannot be reversed or changed. I believe Primary Machine can do this for you. CZ Custom offers a nice milled-in mounting plate, but I don't think you gain much in mounting height. I think many of us with older eyes are looking for compact carry-optic RDS solutions. Please keep us posted!