You can always try it a few ways and see if different settings actually make noticable a difference. Maybe compromise a bit and push 2100 a little higher and 2800 a little lower and go with it. In...

Cell 1 is seeing way more hits than any other, so why don't you make that cell smaller. The top and right rows in your pdf are seeing few hits so you might as well lower those outer boundaries.
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Nice graph!

Looking at your graph, putting the rpm boundaries around 1500rpm, 2200rpm and 3000rpm would seem to divide the data up. 30, 45 and 60 kpa would also seem to divide it up the other way....