Changing how crit works?

So far I've been doing this manually, but no harm in asking if somebody more familiar with making macros and sheets can figure out how to do this stuff. I'm using the standard roll20 5e sheet for the record. The homebrew we're using has crits deal fixed, maximum damage, plus the stat mod again (so a 1d8+2 longsword attack deals 8+2+2 damage on a crit). Any way to easily work that in without editing every single crit field on every single attack?

Sadly, no. Any damage for a crit different than the standard 5E rule has to be manually set for every single instance. There would be too much programming to enable a universal crit calculator as different tables have different rules, not all of which are even compatible with Roll20 roll commands. For example, while it's easy to write in the crit block that a 1d8 is instead 8, there's no roll command you can tack on to any roll to make it output its max value automatically.

Sadness. Guess I just have to go through my players' sheets and update it. And manually keep it up to date. And do it by hand for all my npcs. Ah well!     

This can be done en masse with API, but there's no way to write to sheets without Pro. Just in case you don't already know how to get your own crit into an NPC sheet, you'll want something like this in the description field for each repeating_npcaction (example being a 1d6 + 2 dmg attack): }}{{crit1=[[1*6 + 2]] tacked onto the end of whatever is already there.

I didn't know that, and that's a kind of hacky fuckery I like. I might use it sometime.

I don't think there's any other way to get custom crit on an NPC sheet without API, is there? But yes... good description :)