GMMAT is right about there being several websites where you can get the formula butyou can figure the horizontal distance by figuring the square root of: The height of your stand squared minus the slant (measured) distance squared.
I, too, was interested in trying to figure it out in my head but after doing some calculations there wasn't that much difference in most cases. I figured that my stand height of 15-20 feet on level ground didn't change the distance very much at all and the longer the shot the less impact the angle had on the distance
It would make a difference if you were shooting down into a deep ravine or steeply uphill.
I have started ranging distances by lazering horizontally across from my tree stand to reference tree trunks at the same height as my stand. If you try this you will see that there really is not much difference if the ground is flat and you are at a reasonable height in your stand.