The diagonal angle of view, in degrees, is = 2 * atan(D/2/F)
where D = Diagonal of sensor, mm
F = Focal length of lens, mm
If you wish to determine the horizontal angle of view, replace D with the horizontal length. Similarly, if you want the vertical angle of view, use the vertical length in place of D.
35mm lens with a Leaf 75 (V=36, H=48, and D=60 mm):
AOV (V) = 54°
AOV (H) = 69°
AOV (D) = 81°
As for the multiplier, is determined by the ratio of the diagonal as compared to the 'parent' frame, where the 'parent' frame is 35mm film for a 1D2, 30D, etc, with a D=43 mm. But for MF, such as a 645, the film diagonal is approximately 75 mm. So, for the Leaf 75, with a diagonal of 60 mm, the factor is 75/60 = 1.25. This means a 645 system based 50 mm lens on a Leaf 75 will behave like a 62.5 mm lens. The 50 mm lens on the smaller sensor will thus function as a slightly longer focal length (more telephoto-like).
However, many reference a 35mm film. In that case, the multiplier for a Leaf 75 is 43/60 = 0.717. This means that the same 50 mm lens on a Leaf 75 will behave like a slightly wide-angle 35 mm lens (that is, 50 mm * 0.717 = 35.8 mm).
The same 50 mm lens on a 500CM square format Hasselblad seems even wider (D = 85 mm; 50 mm * 43/85 = 25 mm), but only if you compare the diagonal view, which is what most manufacturers quote for lens angles of view. That is, the horizontal 60 mm length is the same for one side of the 645 and the 6x6 cameras, so the horizontal (landscape orientation) angle of view is the same for either format when using the same lens.