It isn't an exact science.
I strongly suspected that was going to be the case as for everything in this hobby. I find the mantra of the plants will tell you to be the most reliable but it's always good to have some information.
What made me ask was in my main EI dosed tank after not checking for many months (laziness) I found that my GH was 11, KH 4 and TDS of 320, now if I remember my tap water has no measurable gh/kh and a TDS of around 34 last time I checked and I do 50% WC weekly. Conclusion, there's far too much of something going in. I don't use any hardening products so I have to assume that the GH being far higher than the KH is probably from the mag sulphate so as a first port of call I have reduced that which I only add on WC day. The KH must be coming from the seiryu stone in the tank although when I test at start of week and end the kh has only raised by half a degree or thereabouts. Can't work out how that comes about, common sense would dictate that after 50% WC the KH would half but it doesn't seem to do that rather it drops a degree. Maybe I have more water volume than I think.
So based on the DWI this week after WC I test the tank then add the ferts, test again then probably next morning before dosing again look how much has dropped since dosing the previous day and taking into account that some of this will be fish waste and carbonates from the stone the figure would be how much got consumed is that correct? Obviously the figure I obtain will be slightly higher than in reality got consumed because of the increase in TDS from the other things mentioned. I hear dissolved co2 would also make a difference so probably best testing without, ideally before it was running although I suspect that would be negligible.
The reason I ask is two fold, I have some Rams coming tomorrow and would like to get the TDS down without affecting the plants too much so I'm looking to see where I could get and still maintain healthy plants. Plus I'm going for a wander in the lakes tomorrow to see if I can get some stone and if possible some wood for a little project I fancy. I have a 600x300x300 tank sitting empty in my office with a 450mm long led unit (cheapy off ebay) a Koralia and a 25ltr fermenting bin full of cat litter from some previous tanks. I've been doing a lot of reading in UKAPS about low energy soil tanks and it has me intrigued. The way I see it I throw lots of cuttings out so I'm only a bag of soil and some mesh investment away from seeing what could be achieved without a filter just using my cuttings, nothing to lose. Using the DWI/TDS method will be beneficial in both circumstances I reckon and give me a better understanding of what's going on in the tank albeit just some rough figures.
The best of it was I ordered a portion of DW off ebay as a bit of an impulse buy and never thought about asking in here if anybody had some for sale. Searched for Duckweed Index in the forum first post I came across was Darrel telling someone who throws all his extra DW out that he keeps his in ice cream tubs on the window! Doh
It doesn't end there, I was going to cancel the ebay order but thought no, I bought in good faith and I'm not a tyre kicker so I'll leave it but when I checked the Sellers details I found out they actually live about 40mins drive away from me, double Doh
Just left it getting delivered next week, wasn't worth wasting my fuel
Yeah sometimes it's best not to, you just get bogged down in detail.
I suppose you're better off knowing though even if it confuses you more. I felt the same way when I had just got my head round putting loads of ferts in to make sure there wasn't a lack of anything only to find that too much of one can cancel out the other, every day is a school day in this forum.