if you have tips  spaced 15 inches apart  they will apply  20/15 the amount per acre than the same  would at 20 inches........... if everything else is the same.

Be aware  that the  pattern angle  ( assuming fan tips)     will  mean you want to run  at a different height to achieve double overlap at your target height......  Obviously  double overlap  will occur at a different  height with 15 inch spacing than with 20 inch.