Just sharing what I have been doing to focus XY DLP printers perfectly lately.
You need a glass microscope slide.
http://www.ebay.co.uk/itm/100pcs-Pre-Cl ... 4afa754478
Cheap USB microscope - i picked this one up from china. Works great, especially considering price.
http://www.banggood.com/USB-8-LED-500X- ... 83803.html
PVC tape, thinner the better. White
http://www.ebay.co.uk/itm/36-Rolls-STRO ... Swq5lTrIeg
Then it's not rocket science, stick the tape on the microscope slide. Apply it like a screen protector on your phone, no wrinkles or bubbles. Use a credit card to smooth. Trim to size.
The plate, with the tape over it, goes on the table. Tape side down. Focus the USB microscope on the white tape so its perfectly clear on your PC screen.
Now put the plate on your VAT print area to calibrate focus. White tape side facing down, at the point where the printer projector needs to focus on.
Bring up the calibration grid projected onto the underside of the slide with the tape. Now look at it under the microscope from above and focus until perfectly crystal clear on your screen.
May have to go back and forth between your B9 grid and the slide to make sure the build area is the right size.
Here is a 0.7mm grid in focus at 50x magnification (the 500x magnification is just overkill)
(its with another UV projector, hence colours)
Hope that helps with getting everyone's prints ultra sharp in the XY