There is no hard and fast set % for actinic lighting as it's mostly for our eyes, but 1/4-1/3 of the total halide wattage seems to be average for most. So with 500 watts of halides around 150w or so actinic should give you a a pretty good staring point... you can always work up or down from there (if you are DIY'ing them)... if it's a fixture then you may want to go a little higher... 2x250 halides with a pair of VHO or quad PC actinics makes a good balance.