I believe 645 is 42x56mm and the diagonal is 70mm. 135 is 24x36mm with a 43mm diagonal. 70/43 is 1.63 so 1.63x is your multiplier to go from 135 to 645. The inverse of 1.63 is .613 so that's your mulitplier to go from 645 to 135.
I had an 80mm Sekor C for my M645j that was close in FOV to my Rokkor 50mm MD on my XG-M. There ya go. Kinda like converting miles to km's when comparing 35mm to fsd. 1.6 or .62, depending on which way you go.