If total exceeds 40 then allow other fields to populate?

I think I know what you're trying to do.

The way I would tackle this is by first creating a Document Level function

Tools>JavaScript>Document JavaScripts (You can call it whatever you like)

In this function you will calculate all the required parts.

Here is what the function will look like;

function calcSTFields() {
     var ST1 = this.getField("ST1").value;
     var ST2 = this.getField("ST2").value;
     //Repeat for each ST field
     var ST7 = this.getField("ST7").value;
     var totalST = this.getField("totalST");
     var OT1 = this.getField("OT1");
     totalST.value = ST1*1 + ST2*1 + ST3*1 + ... + ST7*1; /*adds all the ST fields together the "*1" is 
so if a field is empty it will still add them together rather than concatenate them as 0 x 1 = 0*/
     if (totalST.value >= 40) { //if the totalST field is greater than or equal to 40
          OT1.value = totalST.value - 40; //OT1's value is equal to totalST minus 40
     }
     else { //if totalST is not greater than or equal to 40
          //do something else
     }
}

This should calculate all the ST fields and set values accordingly.

The final step is to call this function in all of your ST fields, or "ST7" if the user must fill in all fields accordingly.

(You can check for empty fields if you want but that's a whole extra approach)

Now to call the function;

In your ST field's Properties>Actions Tab;

On Blur>Run A JavaScript;

calcSTFields();

From there you should be all good.

Hope this helps!