belt drive ratios

cog	pulley	belt length in mm	distance in mm
18	40	300	75,5
15	40	295	76,1
15	36	285	76,9
16	40	300	77,6
18	40	305	78,0
15	40	300	78,7
15	36	290	79,5
16	40	305	80,2

The belt tensioner needs to work from 75,5-78,7 mm distance, so the adjustment is going from 75-80mm.

With this design, a 300mm belt can be used with 15-40 18-40 16-40 setup.