Auto populate a field based on a range in another field

I'm creating a character sheet for an rpg and I'd like to have it auto-calculate the values for attributes, modifiers, and skills:

• The Attribute field will be a sum of two numbers, the person's originally rolled number for that attribute and any racial modifier.
• Based on the resulting sum of those two numbers, the Modifier field will fill itself with a predetermined number (e.g. If your Strength ends up being 14, your Modifier will be +2).
• Then, the number in the Modifier field will be used as a base to calculate another sum of numbers, this time for Skills.

I've been able to come up with how to do the sums easily enough, but how to populate a field with a specific number based on a range eludes me.

I am pretty terrible at JavaScript when it comes to PDFs, but I feel like this should be relatively simple yet I've had no success. Any suggestions?

EDIT: Here's what I've got so far

var brTotal = Number(this.getField("BrTotal").value);  
    
if (brTotal<=1) event.value = -3;  
else if (brTotal=2) event.value = -2;  
else if (brTotal=3) event.value = -1;
else if (brTotal>=4 && brTotal<=7) event.value = 0;  
else if (brTotal>=8 && brTotal<=9) event.value = 1;

It works... Kinda. When I enter brTotal as 1 or 2, I get the right values, but if I enter 3 or above, it's stuck at -2. Am I missing something?