Thank you for your prompt reply, but looking at Custom Options I don’t think that actually fulfils what I need.
As I understand it with custom fields, if I set a text box it applies an extra price if they fill the box and not if they don’t, it doesn’t take the value and do a calculation.
What I need is a way for it to take that value and add say £5 if it is between 1m and 1.5m and add £10 if it is between 1.5m and 2m etc.
I know I could do that with drop downs, but I also need to collect the actual measurement so that if the customer orders the blind can be made with their specified width and drop.
If you look on the link I provided and put in various values, you will see if you put in a small blind size it will be cheaper than if you put in a large blind size.