You can try this in your formula field "Discount":

switch true do
case Choice = 1 or Choice = 2:
    0.1
case Choice = 3 or Choice = 4:
    0.15
case Choice = 5:
    0.2
end

You can change the number to text "10%" if you want. If you leave it as a number then you can quickly use it in other formulas.

You can check out this post for more background info and more examples of what you can do from Jacques TUR .

You can also use a table with all possible discount rates according to the category number:

item([0, 0.1, 0.1, 0.15, 0.15, 0.2],Number(category))