The “feed at 1 gallon” idea isn’t how you should fertilize. Sounds like you’re either mixing up a concentrate, feeding that, then watering again with straight water or you’re only feeding the recommended amount of ferts for 1 gallon and then adding many gallons of plain water. You should mix up your feed into the entire amount of your water. So if you’re going to use 20 gallons of water then mix your fertilizer at the recommended rate for 20 gallons. I say “recommended” rate, which really means less then what FF tells you, with the understanding that you’d be feeding more often, but at a lower rate. Your plants pretty good size and why you need a big bucket (drum) so you can mix up large amounts of fertilizer. When you start growing big plants you have to feed them big amounts of nutrients. You can’t feed a big plant 1 gallon of mix (which is only appropriate for a 3 gallon pot).
The fabric pots really dry out quick and 10 gallons seems low in a 45 gallon pot. If you’re at 100+ then you may even have to water them twice a day. Personally I wouldn’t water a specific amount, but water till runoff - whatever that takes. I realize that it’s hard in the fabric to know what that amount is, as a quick water will run out the sides. So you really have to water slowly - especially if the soil dries out. Depending on if you’re roots have grown out the bottom, you can try elevating the pot onto a pallet so you can see when runoff happens. The down side to this is the pot will dry out even quicker. But maybe just elevate for a short period until you get a feel for how much it takes to soak your pots.