I am getting my parts and pieces together for my 7.5 budget pistol build and have a question. Does it matter where you mount the red dot/ holo sight as far as over the upper or over the barrel ?? I don't have my MBUS yet and don't know how much room I'll have over the upper but if I had to I could jump over to the quad rail over the barrel but wasn't sure if it effected accuracy at all ? I've search the pic. thread and saw both although the majority were over the upper ??

You want the buis at the very back end of the upper and the you want the red dot/holo right in front of that basically.

I just wasn't sure I will have  room  with the short platform and was weighing my options in case the " what if " comes into play.

The upper is the same size as every other upper.  Here is my pistol with a 7.5" barrel.  I don't have any buis on it yet but  have the red dot placed so that a magpul rear sight will fit behind it.

I actually have my BUIS in the normal placement. I have a TRS-25 actually on the handguard rail. It's a Seekins quadrail and locks up very tight, with zero play anywhere and tabs to align it to the upper.

I have an LMT MRP and started moving the Aimpoint back and fourth until I liked it the best. Ironically, directly over the chamber (where the receiver rail normally ends). With the MRP I put it there because it worked best for me, and I totally could without worry.

With the pistol build, I like it just onto the handguard. Do that test looking through the optic. If your HG locks up very well, then I think you can get away with it.

Mounting it on the upper is ideal, if it isn't monolithic.

Buis as far back as possible, and red dot as far forward as possible WHILE STAYING ON THE ACTUAL RECIEVER.

You could probably get away with putting yor optic on the hand guard but it is not the best place for zero retention especially if you use your gun hard and knock I around alot IMHO

I have no BUIS but I put my EOTECH 552 where its rearmost point was directly over the uppers brass deflector. I run an 8" barrel in 300BLK and this my setup for very short range, thus no BUIS.
I found this to give me the best field of view without my feeling like I was crowding the sight or reaching to see through it.
I also had to use a riser to keep my neck and head up.

Same here.  All my rifles that run red-dots are configured the same way.

^^This

I run mine all the way to the front of the receiver.  I avoid the handguard only because I like to remove them for cleaning every 1k or so rds.