Creating weapons with a proc rate? Possible Macro Help

I am running a 5e game and we are using the official Roll20 Character sheets. I want to introduce some magic weapons that have a chance to proc extra effects or damage. Example, a sword that has a 20% chance to ignite and do extra fire damage on hit. I could set the crit range for the weapon to 17 and put the fire damage there, but that would remove the standard crit on 20. Any ideas on how I could implement this type of thing?

Why not simply add it to the weapon's damage 2 field [[1d100<20]]*X Where X is the amount of fire damage

You can use the the multiplication property of zero and the Identity Property of one to sorta make a type of if/then statement. + [[ ( weapon proc damage roll) * ([[ 1d100<20 ]]) ]]   where "20" is the percent change you have to meet.  The 1d100<20 will return a 0 or a 1 depending on whether or not the d100 roll is higher or less than 20.  This will effectively roll your extra damage, but multiply it by one  (making it equal to itself) if a d100 roll is 1-20  or multiply by zero if it's 21 or higher (making the extra damage equal to zero) You can make this easier to adjust by making the "20" an attribute on your character sheet and referring to it by @{AttributeName} or @{YourCharacter'sName|attributename} Whether the 5e sheet will let you even do this in the built-in weapon field is another story.  Not sure if it will or not without testing.

That is pretty cool! I didn't know that you could place things like that directly in a damage field.