Ossage is right. I dont remember where I read this in the past, but " always use the true horizontal distance." I have used this for many years and it has proved to be very accurate.
__
ll
ll <--- tree stand or up/down hill
ll
ll
ll
ll
ll 10 yrd 20 yrd 30 yrd
ll_____________ l______________l______________l
<----------------- true horizontal distance ----------->

(this is a feeble attempt to demonstrate)