You prolly don't need this but I'll throw it out there anyways, for others...
6.3-6.6 is a little high for hydro, let it drift from 5.8-6.2 during veg, 6.0-6.4 during flower. If you really want to blow them up, the size they are at, they could easily handle 600-800ppm (daily). For moderate growth, mix to whatever strength you're running, then top off with plain water every day for a week, then mix it back up to your number. I top up with water and nutes every day for turbo growth.
For topping to say 800 ppm & I'm at 600 & my res is 4gal low, I'll mix a concentrated batch per Green Leaf calculator as if for say 6gal. Their numbers per their calculator come up to lower ppm's than I like, so I mix my concentrate for extra gallons (to keep the correct ratio) to compensate. I mix it in a 1gal pitcher with a cordless drill and paddle type paint stirrer. Once concentrate is mixed add to your res until at desired numbers. After just a short while, you'll know instinctively about how much to mix and add.