The reason you can dose zero to 15 tsp a week is because the guide assumes nothing about the Mg content of your water. How can we possibly know what the content of anyone's water is when, for example you don't even know what the value is for your own water? Secondly, a trace element is exactly what the word means; Trace. That means you only need a trace amount. So that's why the minimum number only has to be a non-zero number.
The guide assumes, if anything, worst case, i,e, that the hobbyists is using RO water, and that the water's Mg content is zero. Unfortunately, Mg is only a very small percentage of MgSO4. Furthermore, MgSO4 is not sold as MgSO4. It is sold as MgSO4.7H2O which is refered to as "Heptahydrate". So, because this molecule is hydrated, the percentage of Mg in this hydrated molecule is even smaller than if it were the "anhydrous" form of MgSO4.
The long and the short of it is that 10 grams (about 2 teaspoons) of Epsom Salt only has 1 gram of Mg. That's why when you assume the water has zero Mg you have to dose so much of the powder just to get a small amount of Mg. If we change the assumption and assume that your tap water already has some Mg in it, then it becomes much less important to dose large amounts of the Epsom Salt. Then of course, there is the added consideration that adding Epsom Salts increases the TDS/conductivity of the water and some people prefer to minimize the impact of GH due to special consideration for their soft water fish.
You need trial and error because of the ambiguity of the water report as well as the fact that there is a huge diversity in water nutrient content. Even in hard water there is a possibility that the GH content is 100% Calcium with no Mg. It's also possible that the GH is comprised of 100% Mg and zero Calcium. So it's not that the dosing method is contradictory, it's that we don't know what's in your water and one size cannot possibly fit all.
When you experiment with trace nutrient changes, normally 3 weeks is a good period to see any changes. That assumes of course that the changes you see in the plants are only due to the change in Mg dosing and not due to some other factor.
EI encourages you to make changes to the dosing levels and to see the effects. It is an interactive procedure and does not rely on one size fitting all. What you see as "finger in the air" we see as systematic, flexibility and optimization.
See more details and calculations in the thread
Epsom Salts | UK Aquatic Plant Society
Cheers,