r
<- h ->
x
Planet at 125 star diameters
Star
Visible Jump Sphere
Stars 100 Diameter Mask
Surface area of the spherical cap masked by the star is the masked area (2*pi*r*h)
Divide by the surface area of the visible jump sphere and you have the masked fraction (divide by 4*pi*r*r)
The whole thing simplifies to (1-cos(x))/2 where x = asin(100/planet distance in Ds). e.g. 20% mask above