Why I made this
I mix in whatever's in reach — a few watering cans, multiple spray bottles, mixing jars, all different sizes. Most of the time I only need enough for a handful of plants, not a big batch. Some things can't wait around either: AzaMax, for one, has to be used right after mixing — you can't save it for next time.
Recalculating the ratio every single time, or scribbling the math on painter's tape stuck to the bottle, got old fast. Now I type in the ratio from the label, pick the container I'm actually holding, and it tells me how much to add.
Feeding on the safe side
Here's something a lot of growers do on purpose: they want the exact label ratio so they can deliberately go under it — half strength, sometimes a quarter — to stay safe. There's an old feeding rule of thumb, "weakly, weekly": keep it dilute, feed little and often, instead of hitting a plant with one big strong dose. When I'm not sure, I'd rather under-feed than burn. The calculator makes that simple — work out the full amount, then use less on purpose.
Why not just eyeball it?
You can — I did, for a while. It's also how I burned a lot of my sensitive plants. Pesticides are less forgiving still — too weak and it doesn't work, too strong and you might hurt the plant.