The general rule is:
-- throw away the "1/"
-- divide the "From" scale number by the "To" scale number.
The problem is that most copiers (I have used) only have whole percent reduce/enlarge, so in the examples above you can do either 72% or 73% but not exactly 72.92% (you'd do 73% here). And some even have a limit as to how much they will do in one pass.
In some cases you can do a two-step change that gets you closer. For example, assume a copier only did 10% increments & you wanted 25%. You could do 50%, then do 50% of that & have 25%. At the 1% increment this may or may not be worthwhile - the larger the item, the more that round-off to a whole percent might mean.
This two-step approach is more useful if a copier limits the amount of reduce/enlarge.
Sorry, you only asked what time it was, not how to build a watch ...