Id suggest that the OP has a look at Ralph Lambrecht's Darkroom Magic site as well. Ralph very kindly makes available his research into dual filtration and balancing exposure. It is the case that the Ilford dual filtration figures differ slightly from Ralph's own if I recall correctly but more importantly Ralph gives fstop corrections( in fractions of fstops) needed to achieve balance as the dual filtration numbers fail to achieve exact balance by themselves although the differences over say grades 2.5 to 3 are fairly small.

Of course the whole thing is easier if you have an fstop timer such as the Nocon, RH Designs or Darkroom Automation but it can be done with a timer working in secs and tenths with the aid of Ralph's table. Also available at Darkroom Magic

pentaxuser