I add nutes and such when I add water to my res.
Remember- I'm new at indoor growing, too. So anything I pass along isn't necessarily right.
I add nutes when I add water, pre-mixing the solution in a seperate clean bucket. This is where the ppm tester is so very imprtant. I started out adding my nutes directly into my res, checking the ppm's and adjusting as I went along. I very quickly found that this was not a good idea. Say my 3-1/2 gallons of solution is around 200ppm, by adding directly to the res.-1/8 tsp- barely a couple drops- will usually raise it by 50ppm+-. If you happen to slip up and add a little too much, the results can be devastating. Not sure of what your growing, but as you probably know, some types are VERY nute sensitive.
So, to avoid that, I mix in a seperate res, nutes first then adjust my ph. I've started mixing everything the night before and let it stabilize overnight, re-check and add it to my res in the morning. Seems to be working out okay. With a little math and averaging, I can mix the solution a little stronger each time to gradually increase my total ppm's without fear of accidentally overdosing. I mix a gallon at a time, but you could mix 1/2 gal at a time with a tester.
1 of my babies is nearing the full vegetation stage, and is sucking up between 1 1/2 - 2 pints of water a day! I expect that to double, if not more by the time she's in full bloom. I've heard some say theirs can drink up to a gallon a day when they get big, so...
I also learned- very important- that if you mix a gallon, and only use 1/2 of it, use the other half on your tomatoes in the garden or something. I store mine (short term 2-3 days) in the refrigerator where it's cool and dark. If you leave it sit out, whether in a jug or bucket, it will quickly begin to form green algae, and you can't use it. Light and warm temperatures can really work against you.
Keep a daily log book, if you're not already. I use a cheap day planner and have 1 column for (daily) ppm start, ppm finish, ph start, ph finish and a space for daily notes. Each morning when I check everything, I enter all of my start values, make any changes or adjustments, then enter my finishing values. Keeps me from trying to go from memory- 'cause I was probably stoned that day. I have 4 buckets with 3 varieties going right now, so it just helps me keep shit straight.
Best of luck!