Anyone using a later edition Ilford Multigrade 500 heads will be used to using blue and green light for split grade printing, and I think they have been quite effective .

And as for intervals, have you considered using logarithmic (e.g. 2, 2.8, 4, 5.6, 8, 11) instead of arithmetic ones (5, 10, 15, 20, 25, etc.)