Macro Help: Keep lower value for math between Attribute and flat number.

I have a boss enemy that has a unique damage situation. Basically you get up to a certain amount for resistance to their damage. (Their damage cannot be reduced by more than 20). What I am trying to do for my DM is create a macro that does the math without having to pull up character sheets to use with NPCs (this is a big final battle sort of deal). Essentially I am trying to do this math wise:  Damage minus resistance(if it is less than 20) OR Damage minus 20. But like in one command function. I know keep lowest is a thing, but can it be used with attributes and flat numbers or only dice? 

I actually figured it out through some serious digging on the forms and found a four year old post that I think will work for me. Keep lowest was correct, but how I was inputting them was wrong. For anyone else that for whatever reason needs this topic answered its: [[ {[[@{stat}]],[[20]]}kl1 ]]

I'll just take a moment to let you know that the "keep low" and "keep high" function compares either two flat numbers or two rolls to each other. If you want to compare a dice roll to a flat number, just turn the flat number into an XD1 roll and voila ;oP