How many XR15 for 5.5-6'

[shiftline]

With the xr15 the normal lens does 18" and wide is 24".

Assuming I use the standard Lens with 18" spread how many would you think I would need for 5.5' (maybe 6')? With 3 it would give me 6" of unlit on either end. With 4 there would be A lot of overlap?

Any suggestions as in planning a future build.

The tank will be 18-20 wide by 5.5-6' long and 18 high