Link parameters to the Checkbox Control effect.

checker = effect("Checkbox Control")("Checkbox");
on = 100; // Value if enabled
off = 0; // Value if disabled
if(checker==1) {on} else {off};

Alternate checkbox – expression, checks if the layer or effect is on:

fx = thisComp.layer("Settings").effect("Wiggle") // Name of the layer or effect
on = 100; // Value if enabled
off = 0; // Value if disabled
if(fx.enabled == 1){on}else{off}