To Calculate actual Vertical offset within vertical keystone spec
tan-1(The difference between height of the top of screen and the height of the projector divided by the distance from screen to projector)
Example:
Top of screen= 79”
PJ height is = 96”
96”-79” = 17”
Difference is 17”
Distance from screen to PJ( which is given by the image size and throw distance chart) = 135”
tan-1 (17/135) = 7.2 degrees
Correctable with vertical keystone adjustment ( Vertical Keystone correction is 15 degrees max)