i am think you are using the qty field for meter, that is wrong.. qty is for pieces…
you should use custom options!
admin: catalog, manage products, select the product you want.. then click custom options at the left.. add a new one..
Then set up the options… by clicking the ‘new row’
Title: 3 meter
clicking the ‘new row’ again:
Title: 5 meter
in the front-end the user have to give also the length and pieces…
if you want the customer to let input any numbers then you should use text instead of dropdown…
in that case you have to hard code some files, to change the price…