Did some testing today.
I checked substrate pH and the top was 6.5 and the bottom 6.5
not good. I did not have my pH tester the last few days.
Another interesting observation I had was with EC.
I flushed on day 44. After flushing the substrate TDS was 127 (ECX500) and substrate pH 7.0
After feeding (RES TDS 790 and pH 5.0-5.5)
Day 44
SUBSTRATE TDS
top: 370ppm pH: 6.5
bottom: 500ppm pH: 5.5
RES AFTER FEEDING
775ppm pH: 5.5
during this week I adjust res, top off, add additives etc...
Day 52
Before feeding (with minor adjustments)
RES
880ppm pH:5.0
SUBSTRATE
top: 1000ppm pH: 6.5 for tds mixed top and bottom
bottom: 1000ppm pH: 6.5
SUBSTRATE AFTER FEEDING
top: 440ppm pH: 6.5
bottom: 940ppm pH: 5.5-6.0
RES AFTER FEEDING
TDS 880ppm pH: 5.0

I used to only use 100ppm of cal-mag. I noticed a bit of mag deficiency so I started using 250ppm. This fixed the mag deficiency but it is making my pH rise pretty quick. I am in the process of figuring out just the right amount of cal-mag to add. The way I am going about it is when I change my res I try to balance the pH of my base nutes and cal-mag to around 5.0-5.5 It has been hitting that range with around 250ppm of cal-mag at this stage. My base nutes are around 600 When I only added 100ppm of cal-mag my pH was too low. (I use a silica additive to raise my pH) Now at 250ppm my pH is fairly stable. (my plants miss there silica, its ok more ppm for other stuff)
The way I go about adding my base nutes is I check the TDS before and after i water. If there is a huge drop in TDS I bump up the ppm of the base nutes with the appropriate balance of cal-mag. I add my supplements a few days after res changes. I do this a few days later just to make sure my cal-mag and base nutes are in balance. This is how I have been going about figuring out how to feed my plants. So far this method has been working really nice. I do not know if it is the right way to go about things but my plants are really happy and almost ready for harvest. I like this method I am using because I am able to tweak a custom feeding schedule for each strain I grow. Another thing I notice is it is not far off track of the feeding charts from various companies. Right now I am at day 52 using 250ppm of cal-mag and around 600ppm of base nutes. This is all figured out just by stabilizing the base nutes uptake (by checking ppm before and after) and stabilizing my ratio of cal-mag to base nutes (by checking pH). I have no idea if this is a good way to go about things but my plants are happy.
The pH and TDS stuff I posted does not really apply to how I go about fixing my nutrients. It is just an example to keep an eye on your substrate and not just your res. Things can get messy by those roots. I notice after a flush the TDS of the substrate slowly increases each day. When it gets above 1000ppm I flush. This happens about once a week to once every two weeks. (I need to flush now)
Just thought I would post how I do my nutes. I have never read about this style anywhere. People usually follow a chart or use 1/4 strength then bump it up to 1/2 etc... My method is all about stabilizing (and of course what my plants tell me). I am not saying it works any better than any other method. It is just my style. suggestions?
my plants seem to be happiest when TDS is stable to a slight drop after waterings.
I water once a day. It might be beneficial to fit in another watering. As the plant drinks the substrate starts going a bit mad. Once a day has been nice so far. I might aim for twice a day next grow.

my starting water is around 40ppm.
Thats tap. The bay area has some good water.