Change field from read only and override calculation based on radio button

Hi,

I have a form in which one of the fields has a simple sum calculation and is set to read only so the user can't change it as it sets a print amount as well.

But if one of the radio buttons is ticked there is a chance that the print amount needs changing but it can be different based on a few other options so I have managed to set the field so the user can edit it depending on an option but the calculation that sets the value overrides what the user enters. I would like to know if it is possible to change this?

I use

this.getField("Quantity_Boxes").readonly = false;

So the user can edit the field when they select the radio button 'yes' under Mesh_b

and

this.getField("Quantity_Boxes").readonly = true;

So the field goes back to read only when the user selects the 'no' under Mesh_b

and in the Quantity_Boxes field I use:

Quantity_Curtains / csn

Just as a simplified division sum with the decimal places set to 0 so a whole number appears.

csn is just a hidden field that has an amount set depending on the size of curtain.

Thanks,

Bruce