Field value to determine which checkbox is selected

Hello,

I'm trying to write a script to have one of four check boxes selected based on the value entered into a field.  I have pasted a picture of the portion of the PDF that I'm trying to write the script.  I want the "Grand Total" field to be the trigger so I put the following code into the custom validation script of this field.

this.getField("unqualified").checkThisBox(0, (event.value <="23"));

this.getField("marksman").checkThisBox(0, (event.value >="24") || (event.value <=28));

this.getField("sharpshooter").checkThisBox(0, (event.value >="29") || (event.value <=35));

this.getField("expert").checkThisBox(0, (event.value >="36"));

I'm unsure how to write the code for a number range (i.e. 24 - 28; 29 - 35) therefore multiple check boxes are selected using my code.  I need only one check box selected based on the "Grand Total" field.