So the Miracle Gro All purpose fertilizer says to use a gallon for every ten square feet and gives me the dilution rate. I get that. However, a gallon of water isn't enough to fully water ten square feet of soil. Do I a) keep mixing up new gallons to finish watering, or do I b) top it off with just plain water? A seems like it would add too much fertilizer and B seems like it would overly dilute it.
While I have chosen to avoid the stuff for years [and we'll leave the differing opinions at that], your problem is easily sorted.
You are confusing "the application of fertilizer"
They are not the same process.
Water adequately, or just under adequately.
Then "apply fertilizer" by spreading 1 gallon of mix across 10 square feet.
If you "overdilute" or "fertilize and then water (if your plants need serious irrigation)" you may wash your soluble fertilizer right out of the root zone before it does any "good." "watering in" dry type fertilizers would again be a LIMITED application of water as part of the "fertilize" step.
I would just dilute more. The plants have some needs some elements to build up their organs (usually some milligram or some grams of N, K, P, etc.).
So ten square feet needs a quantity of fertilizers. You can dilute more.
Note: usually it is best not to use less water, because too high concentration could be toxic, it could change acidity, or just make difficult to the plant to absorb it and distribute to other cells.
Too much water could bring away good substance, especially on sandy or rocky grounds, but I don't think you will use so much water.
Additional note: the fertilizer seems a "all purpose", so possibly it is also over-fertilizing (but if you have a demanding ground), so I would have no problem on adding more water.