Good morning, mercime. Aluminum sulfate comes in several forms (powder, etc) and in several concentrations so a single formula will not work for all versions sold out there. I hate it when they give me weight requirements like that (because I have no scales of any type) so, what I do is determine how many pounds I will need as a percentage of the package's weight and take out that much. So if I need to apply 1 pound and the package weighs 4 pounds, I extract 1/4 of the package's content. Because less is more when dealing with compounds that have Sulphur, feel free to use less anytime.

Because my soil contains naturally occuring aluminum but is alkaline, I always have to apply something in Spring and sometimes again in the Fall. My bag of a/s is now empty since last Fall and I am just using Garden Sulphur. If your soil has adequate amounts of aluminum, you can do the same; otherwise stick with a/s. You can also use iron-chelated compounds or greensand if you are into organic stuff.

For a 15% concentration of aluminum sulfate - the most commonly sold formulation - the recommendation is 1/4 cup per foot of plant. As too much Al2(SO4)3 can create aluminum toxicity, it is better to err on the low side :-) Should I need to apply AS, I would do it half strength but at a couple of different times (fall and late winter/early spring)

The 1-2 Tbsp per gallon of water is a diluted formulation, obviously. If you were applying it directly to the soil (without diluting it in water), you would likely be applying at the 1/4 cup rate as well.