Speaker Set Up...Math/Geometry Experts Please Help

Hey guys, I need your help. I am helping a friend of mine set up his speakers and can't figure something out.

His speakers need to be tilted back to time align the drivers. According to the graph supplied from the manufacturer, we need a rake (tilt) of a 1/2 inch. This is measured using a plumb bob hanging from the back of the speaker, (which is perfectly flat). You measure the distance from the string to the back bottom of the speaker. The problem we are having is that even if we let the plumb bob settle, as soon as we try to measure the distance, the string moves just from the air disturbance around the string. We are having trouble getting an exact reading.

My question is: Can you convert the 1/2 inch distance to a degree? I have a really nice digital level that we can put on top of the speaker, start with it perfectly level and zero out the level. My level only reads in degrees, so I need to know how many degrees correspond to 1/2 inch tilt.

I've actually done this a few times in the past and always used the plumb bob method, but with current technology, was hoping to get it perfect.

Maybe this can't be converted? I've tried looking it up online, but Geometry was always my weak subject.

Thanks guys!