It seems to me like you should be able to set a baseline date, and then compare against that. You’re setting 100% to be the highest point ever, and then marking down from that. But that won’t show growth since the specified date.

For me, I’d want to go back to the date that e-mycin was released. Or maybe Eliza. Perhaps you could predefine some known benchmark dates to compare against? Something that ties to a specific event, as opposed to “the last 10 years” or “the last year”, etc….