I'm glad you have time to tinker and get it right.
I've been playing too, I have a dutch bucket hydro system and have a PPM meter so I did some testing. I hope this makes sense
I bought some bulk MG, it's blue compared to the green stuff in the bottle and twice as strong. The blue is 24-8-16
The bottle refills are 12-4-8
The MG Tomato food is 18-18-21
I'm not exactly sure how PPM changes with different strengths but it took 6 tbsp of the blue MG mixed with a little water to reach the 450PPM that matches the prepacked bottle.
The instructions on the blue stuff
I went out and started testing all the mixed solutions
I set up the feeder like directions say for the feeder and tested the supplied bottle and out the hose the PPM is 519
I then mixed a batch of the blue stuff, 1 tbsp-1 gallon of water and it tested 885, this is closer to what my hydroponic mix is but maybe it's too strong for the garden to feed more than once a week because we already have most of the requirements to grow compared to hydroponics soil-less mix but I'm just guessing.
I changed the solution in the bottle and ran that home mix of the blue MG I mixed earlier, it was only 186. This should confirm that the fertilizer in the bottles are made from different stuff and I don't have a meter to test so from here I'm winging it.
I mixed another 4.5 tbsp and then it tested 265
I added another 4.5 and it went up to 366
That's a total of 15 tbsp to get 366, I'm guessing another 6 tbsp and it would reach the same PPM as the bottle but about half as much as the recommeded mix in the directions.
I used 1/2 lb of the 1.5lb box I bought for $5.00, that $1.66 for 40 gallons
I haven't been using MG so I can't say if works good mixing it lighter than recommended. I'm sure that most of us amend the soil before planting and that should help to not need a full strength mix as a weekly feeding.
I think it'll work great as a light feeding once a week and I'm gonna start off with 10tbsp and see how it goes. Thoughts?