really depends on the bridge model you're using...
...I also have a D-Sonic on my RG550 and have a Breed Neck for it, those 2 seem to balance each other(390mV & 330mV respectively) volume-wise; on the other hand, my JEM777VBK has the stock Paf Pro and an Evo (<300mV & 420mV) and those seem to even out pretty well. My last experience was matching a D-Activator X with a Paf Pro (500mV & 300mV) and I had a REALLY hard time matching the volume level of the Paf Pro to the D-X.
Essentially, the general rule of thumb for HH/HSH setups, bridge pups are mounted much higher than the neck pup

But so far, experience tells me 60mV – <150mV difference would be a safe bet...
Reg