How to do this using custom script if user select box uom it will show into the nos uom in custom filed?

Hello Team,

I need to show 2 type of uom conversion for a single item in my sales order .
my main Uom is box but i need to show how many nos in that box .so for that i have created one custom filed like box_to_nos_conversion.what i want when user select the box uom it will automatically convert the nos based on the box and add in to the custom filed .