ETC has an app for that- the Cyc Tool on this page:
The ETC Cyc Tool won't steer you wrong. You need to know what setback(s) you will install them at, and I suggest keeping the "horizontal blend" slider on the "best" half of the scale and get the most you can afford within the range the tool spits out (gives you flexibility with placement, and they are really handy compact fixtures for set washing and such too). You'll be very happy with them for a 16' height. My guess is you'll end up with between 6 and 12 across.