Over the years I've made a bunch of spreadsheets and scripts to help with DMing and understanding things like challenge ratings and contested checks.
I'm slowly making them into web apps in the hope that other people find them useful.
New one released today: https://ccodds.whitebeard.blog I made this to understand how proficiency, level and advantage interact in contested checks. You might find it useful, let me know.
ccodds.whitebeard.blog
Never tell me the odds