Can you clarify:
Are you adding phyto and then adding Miracle Grow to your tank directly?
Are you growing your phyto in RO water with salt or storing phyto in RO water?
You can overdose phyto, but since you're growing it yourself, it's live (mostly) when it hits the tank and should last longer in the water column with much less polution. You don't have any primary livestock that will consume phyto so you shouldn't really need to dose it much at all. A little bit will go a long way in a pod poplulation. Overdosing will potentially cause buildup problems, but as most will stay in the column, your skimmer should remove the excess unless you're dosing faster than it can remove.
You can help remove any remaing urea and chlorine by using prime on phyto batches you plan to feed with (don't put it in with the stuff you plan to continue growing). Also, you could let it grow even longer (say 3-4 weeks) per batch since you don't need it that much and that will deplete the fertilizer more.