Take an example of T5HO high output tube in conjunction with a MVB over the basking spot , tube being 75% the length a tank which is fine PROVIDED it has one end right next the MVB ( behind or infront of it like this PLAN VIEW )
If it's like this
Now consider a example where the tank has the dimensions of 24" H and 26" D :
A picture says a 1000 words and shows everything succinctly and clearly.
It is likely the UV spread from the end of the T5HO will create a low UV zone (perhaps even a zone at very close to zero UV). If the UV tube is more than 12 inches from the cold end of the tank , the UV in the last 8 inches will be very low (near zero).
This can be confirmed by looking at the iso-irradiance charts published here
http://www.uvguide.co.uk/fluorescenttubereflectortests.htm
that look like this :
or using your solarmeter I'd recommend measuring UVB flux , but if you have a solarmeter UV-Index meter , you can test the UV intensity at the low UV area near the end of the tank furthest from the basking spot.