I look at the smallest gear I need and the largest. Say 42-32 and 53-12. Convert those to gear inches. (42/32 X 27 =) 35.4 GI and 119 GI. Now you can look at the chainrings you want to use and see what cassette results or vice versa. Say you really like the spacing on a 11-28 cassette. 119/27 X 11 = 48.5. 35.4/27 X 28 = 36.7 So a 48-36 by 11-28 will be near identical to a 53-42 X 12-32.
Or say you really want a 50-34. So 50 X 27/119 = 11.3 and 34 X 27/35.4 = 25.9 A 50-34 by 11-26 will also get you the same gearing.
Actually, after I decided to go 9-speed years ago, I purchased several different cassettes and a couple of outside cogs so I can have two wheels built up in nearly any combo from 12, 13 or 14 to 23, 25 or 28. (I've used 53-42-28 chainrings for decades.)
Ben