java script help

I am tryin gto get the numbers entered into one field to populate into another field.

The number in field 1 would always need to round to the nearest multiple of 4 then divided by 8.

I have the current java script which works to populate whats in field 1 into field two.
But how do I get the numbers in field two to further calculate as per above?

I have

//Set the vars one and two
var one = this.getField(“Hours1”);
var two = this.getField(“Converted1”);

//next check if two is blank and if so, polulate it with one’s value
if(two.value==”||
two.value==null)
{two.value=one.value}

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