Is there anywhere that shows how to code the fixed value for homebrews?

For example, I'm trying to code this into a Feat:

When you make a successful unarmed strike or keep an enemy in a grapple while wearing no or light armor, you may also deal additional damage equal to half your Constitution modifier, rounded down (minimum of 1). The damage type depends on your Gel-Type (Glacial—Cold, Lava—Fire, Ochre—Acid, Plasma—Force, Pudding—Poison).

You mean the “half the constitution modifier (rounded down)” part? That’s not a “fixed” value since Con mod can change. A fixed value would be something like “3,” fixed because it would always be “3.” (That’s what fixed means.)

Not like that, no. "Keep an enemy in a grapple” is just a Strength (Athletics) check, and the sheet would have no way of knowing what any particular Athletics check would be for, or if it was successful. And you can’t add a bonus based on half of a modifier to anything. You could add a fixed value to Unarmed Strike damage, but again, a fixed value wouldn’t scale with Con.

What you could do is simply use the snippet to calculate that and display it in the txt on the character sheet. The player would have to remember it every time, but it could automatically calculate it for them:

Whenever a creature fails to escape your grapple, or whenever you deal damage with an Unarmed Strike or natural weapon, the target takes {{(modifier:con/2)@rounddown,min:1}} addition damage of a type dependant on your Gel-Type (Glacial—Cold, Lava—Fire, Ochre—Acid, Plasma—Force, Pudding—Poison).

That snippet should be correct. (I think.)