Optimal Placement of LED's
I think what you are interested in is what this BIVAR datasheet calls:
Relative Luminous Intensity vs. Radiation Angle


I think it is hard to keep the relative luminous intensity exactly constant along all points in x and y at the fixed Z of the board as I suspect this may not only be affected by the incoming luminous intensity but could also depend on how the different materials/components on the PCB react (some may be reflective/cast shadows/different heights).

You might want to consider some sort of lens (such as this spherolit lens) which changes the dispersion properties.

http://www.ledsmagazine.com/content/dam/leds/migrated/objects/features/8/6/7/ERCOfig1.jpg

In short, I think if you can:


Use the datasheet of the item to arrange the "edges" of the coverage of LED emission
(imagine drawing a line from left to right on the top diagram to find a sweetspot where the maximum amount of radiation angle is covered)
Consider using some lens/dispersion technique


Then you will be going along the right lines.

Perhaps if you had some measurement/test equipment (such as a voltage outputting UV sensor) so that you can experiment with different UV LED arrangements and measure the UV intensities using a grid of fixed points on that Z plane this will help evolve/refine your design.

Here is a link to the page containing the BIVAR datasheet with information in it:
http://uk.rs-online.com/web/p/visible-leds/7134888/

Good luck!