How is the chamber lead angle determined?

[craneman]

Can anyone describe how to measure the lead angle of of a chamber cast?
I have a new project on the bench, it is supposed to be chambered in 44-100 Remington straight and I am hoping to build this into a dedicated long range paper patch rifle. There have been a number of people changing lead angle to 7 degrees for paper patching and would like determine what I have so I can see if there is anything I might want to change before finishing this build.
It is sporting a #4 Green Mountain barrel with 1-17 twist on a high wall action.

   

Thanks for any and all input!

Todd