I have a Microsoft Excel spreadsheet that will do the math for you. It converts a tube’s center-to-center measurement to the distance from the edge of one tube to the edge of the other. If you email me a request I can send it to you. My email address is my name in all lower case letters without a space between my 1st and last name. After the symbol you get by using the shift key and the number 2, the rest of the address is qtm.net.
This spreadsheet was done for me by one of my framebuilding class students many years ago that took engineering at nearby Notre Dame University. It has a page for a top tube and another for the down tube. You have to fill figures in 6 columns: What is the length of your tube? What is the head angle? What is the seat angle? What is the head tube diameter? What is the down tube diameter? What is the seat tube diameter? Once you fill in those dimensions, the spreadsheet in column B will fill in the top edge-to-edge distance and in column C the bottom edge-to-edge distance. Of course you will get your original center-to-center dimensions by measuring them off of your full-scale drawing.
In recent years I haven’t used this spreadsheet since Bikecad and rattlecad became available but it is still a good option for those that don’t have those applications.