has any one put a doctor optic on a USP?  

I did, my Tac but took it off as it has a hood which requires time to remove, till then your sightless.

For sighting, for me, I had to draw, raise the end of the barrel, lower it till the dot came into sight then fire.  I would suggest the 7mm dot, I have the 3.5, kinda fine at arms length.  Brightness was good on a sunny day in the snow.

When my shots would start to wander, the sight needed tightening, then resighting.

I did like it, but it takes a little getting used to, great for older eyes.