If you are set on using a dichroic head then have a look at Ralph Lambrecht's site "Darkroom Magic". He very helpfully gives correction exposure factors when you need to change contrast grades. I have never found the Ilford or anyone else's dual filtration to give a constant exposure when changing grades although the exposure differences are relatively small over say half a grade say grade 2 to 2.5 but beyond this the exposure difference is enough to give a different print from the one you'll get if you stick to the same exposure