A = 100*(yourscreenswidth-yoursprite.x)/yourscreenwidth
Since your hotspot probably isn't on the right side, you have to, for example, also add half of your sprite's width, if it is in the middle of it.
A = 100*(yourscreenswidth-yoursprite.x-yoursprite.width*1/2)/yourscreenwidth
Ofc the formula needs further tweaking, if you have a moving screen, that doesn't start with x = 0 to the left.