You put a straightedge across the rim and measure from that to the wheel mounting face, don't worry about where the tyre fits. Only wheel width is measured bead to bead.
“This ain’t no party, this ain’t no disco, this ain’t no foolin’ around”
I've been bitten by this and in my limited experience found it differs from manufacturer to manufacturer. My Cragars are measured as Blue suggested, however my Wheel Vintiques are INNER of the rim to mounting face.
Yep, 1/4" here and a 1/4" there can make all the difference ! I was lucky enough to get away with it but it all depends on how much room you have to play with !
I stuck the slicks on a borrowed pair of rims and at the moment I do not have 1/4" space twixt tyre and car, so once again fortune has vomited on my Eider down; hence the post!
I know this thread is old now, But can you get scrap tyres to experiment with.
I have some spare wheels(mags and cop rims) if that helps, bit of a trek to get them thou