You could use the belt calculator web for finding the right belt https://www.bbman.com/belt-length-calculator/ , i asume you know of it, but anyway here is the link.

it seems that 17-18T is better choice as it has 6 teeth in mesh, versus 7 teeth for 20t, so you lose almost nothing. 6 teeth is the minimum recommended for HTD 5 belts, so you are ok there.


so 17x7=119t, which is better as its a whole number of ratio . Its not bad 1:7 ratio . Its 180mm diameter puley, the big one

I can generate you the pulley profile if you need. Thingverse also there is a pulley generator.