No, it's not possible to specify the interval and/or range; SONAR just calculates what will fit based on the zoom level.
Your best bet will probably be to zoom to one of the levels that gives whole numbers. I just checked, and depending on whether you zoom in from the minimum resolution of 20 BPM per division or out from the maximum of .2 BPM per division, you can get whole number increments of 1.00, 3.00, 4.00, 5.00, 6.00, 8.00, 12.00, 15.00, 16.00 and 20.00 BPM/division.