Obviously you need to do tests to figure out your dosage but that injector fertilizes 40 gallons with the bottle that comes with it. I tested it against the blue MG stuff and it's different from the green stuff it comes with. If you use a weak solution you can feed at every watering. I'll use Masterblend, calnit and epsom this year and I'll have to test it.
I did some test on it last year and posted them somewhere. Here's the results of me testing the blue MG against the green MG solutions. This was to use the original bottle that came with it. It's probably Greek but ask if you have questions.
I set up the feeder like directions say 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 because we already have most of the requirements to grow compared to hydroponics soil-less mix.
I changed the solution in the bottle and ran that home mix, it was only 186. This should confirm that the fertilizer in the bottles are made from different stuff.
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 recommended 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
You can use any water soluble fertilizer, just test the ppm and make adjustments.