39 1/8 is the recommended RTW from Ultramax on the Badmax. A good start is to place the RR tire 1/8 - 1/4 off of the rr cassette hanger and then set the LR as necessary to achieve the recommended RTW. Measure the RTW from outside edge to outside edge of each tire. That is where the tire comes in contact wuth the racing surface.
This is not ultramax website but, it does show the proper messuring point for RTW on a kart. Just click on any chassis set up page and view where they show to get RTW.