I don't know if I can help, but, here's what I do:
I have a six 5-gal bucket RDWC system with a 40-gal res(only uses about 20-gal in the res), The only time I change the res is when I'm changing nutrient ratios or I've gone beyond 6 weeks without a change. Ive read so many different opinions about when to change the res. I've read everything from "I change the res twice a week" to "I never change my res except between grows". so what works for me is about 3 changes for the whole grow cycle. when I top off, I usually just top with tap water. when the EC drops, I add nutes to keep the EC level where I want it for the appropriate grow phase.
example:
my tap water is .45 ec or 225 on the .5 ppm scale
I use the GH 3 part flora series (gro, micro, bloom)
1. for the first 3 weeks of veg I use(on the current grow) 3g:2m:1b at a 1.0 ec or 500ppm (very light on the nutes)
2. after 3 weeks, I bump up the nutes to 1.4ec (700 ppm)
3. at the 6 week mark(start of bloom, light cycle change), I change the res and start with a 1g:1m:1b ration at around 1.6 ec (800ppm), only for 2 weeks.
4. at the 8 week mark(two weeke into bloom), I start using a 1g:2m:3b to increase my ec to 2.0ec (1000 ppm)
5. at week 12 (6 weeks into bloom) I change res and and add add 1g:2m:3b to get back to my 2.0ec (1000ppm)
6. roughly 2 weeks before I think I want to harvest, I stop adding any nutes and just keep topping off with just water.
note 1: I always top with just water and adjust the nutes after topping off the res.
note 2: when I change the res, I'm never empting the whole system, when the res is empty, the buckets still half full.
note 3: I think this cycle, instead of just stopping the nutes on the last two weeks, I'm going to change the res with just water to help the plants use up the stored nutes in the leave more
note 4: keep in mind, I tend to run my nutes kind of light compared to others, but I haven't had an issue with staving plants yet. More nutes does not mean bigger plants, appropriate nutes means healthy plants };-)
I hope this helps a little };-)