Not sure, but can you replace it with a cap that has the window? Sounds like you are doing the adjustment correctly but without the window, the re-check and fine-tune becomes the issue as you have stated.
Also you can go point-less and use a Pertronix kit or do the HEI upgrade or find a distributor that has the window.
http://www.pertronix.com/prod/ig/ignitor/default.aspx
http://6066gmcclub.com/gmcguy/hei.html