I’m wanting to switch UVB lights, my little guy is in a 75 gallon tank and the light will be placed on top of the screen, his basking spot is 10-12 inches high I would say..should I get the 12% or 14% Arcadia bulb?
Get the 12%. I use the 14% in my 4x2x2's and am getting UVI levels near 5 and that's at a distance of 18-20 inches away. At 12, even with a screen, your UV levels will be unhealthy.
The only difference between the 12 and 14 is the effective distance of the bulb. The 14 was made for larger (taller) enclosures.