I know this will be a little controversial but on the second day of flower I topped the highest part of the canopy. I'm hoping this will slow down the growth enough for the second and third layer to catch up.
not really controversial. just a pruning technique. another way is to strip the fans off the top few nodes. both work.
When you talk about adding more fertilizer to an already mixed solution, that's called an addback. In general, 1/4 strength of your normal strength per volume of top off water is what you want to use as an addback.
What you want to pay attention to is how much water you are adding daily. When you add half of the total volume of your reservoir back it is time to change it. OR, if your EC falls unacceptably low.
The reason you do this, is because especially in flowering the plants secrete a ton of enzymes, proteins, and hormones through their roots, "exudates". They don't use these things, and these things accumulate in the reservoir, contributing to the solution's phytotoxicity.
Also, it is impractical to measure individually the elemental content of the nutrient solution. Therefore we have no idea beyond an educated guess as to what needs to be added or when, which is why addbacks are diluted.