Check box runs a script in another field

First off let me begin by saying I am a newbie at JAVA. I have some DBase knowledge from years ago so somethings in JAVA are not foreign to me. My question is... I have a pdf form where i want to runs a calculation script based on a check box. So, I check a box and it runs a script in another field name. I have been searching the web and found some similar scripts but they don't do what I want. Thanks in advance for your help.

This one works filling in the value in the same field

var nFOOTAGE = this.getField("FOOTAGE").value;

if(( nFOOTAGE >= 1 ) && ( nFOOTAGE <= 1500 ))event.value = 250;

else if(( nFOOTAGE >= 1501 ) && ( nFOOTAGE <= 2000 )) event.value = 300;

else if(( nFOOTAGE >= 2001 ) && ( nFOOTAGE <= 2500 ))event.value = 325;

else if(( nFOOTAGE >= 2501 ) && ( nFOOTAGE <= 3000 ))event.value = 350;

else if(( nFOOTAGE >= 3001 ) && ( nFOOTAGE <= 3500 ))event.value = 375;

else if(( nFOOTAGE >= 3501 ) && ( nFOOTAGE <= 4000 ))event.value = 400;

else event.value = 0;

but I want a checkbox (RICheckBox) to activate the above formula in another field (RIP).

I tried this but keep getting an error message at line 2

if (this.getField("RIPCheckbox").value=="Off") event.value = "";

else event.value= var nFOOTAGE = this.getField("FOOTAGE").value;

if(( nFOOTAGE >= 1 ) && ( nFOOTAGE <= 1500 ))event.value = 250;

else if(( nFOOTAGE >= 1501 ) && ( nFOOTAGE <= 2000 )) event.value = 300;

else if(( nFOOTAGE >= 2001 ) && ( nFOOTAGE <= 2500 ))event.value = 325;

else if(( nFOOTAGE >= 2501 ) && ( nFOOTAGE <= 3000 ))event.value = 350;

else if(( nFOOTAGE >= 3001 ) && ( nFOOTAGE <= 3500 ))event.value = 375;

else if(( nFOOTAGE >= 3501 ) && ( nFOOTAGE <= 4000 ))event.value = 400;