Hi Lee,
To my knowledge, with RGB filters the usual goal is to represent "natural" colours of the stars and DSOs. So I thought you would want to have similar exposures for each filter corrected of course for CCD's QE for each colour. ICX 674 has higher QE for green and blue colours than in the red, so perhaps exposures through red filter need to be a tad longer. It also depends on filters transmittance of light as well.
I would recommend finding a right RGB exposure ratios for your RGB filters and perhaps expose for as long as possible with the Lum filter.
EDIT: I think it makes sense to me to avoid saturating stars with RGB filters
