Hi,
There is a misunderstanding here.
There are several custom quantity plugins.
You have installed one which handle the width*length calculations.
If you want to handle length calculations, you would need another one.
However, two custom quantity plugins cannot work together. So that leaves you with having to modifying the one you have to handle the calculations in both cases.
It's not complex. Edit the plugin php file and you'll see this piece of code (twice in the file) normally:
if(!empty($product->length) && !empty($product->width)){
$quantity = $quantity*$product->length*$product->width;
}
You could just add after that (in both places) :
if(!empty($product->ordered_length) ){
$quantity = $quantity*$product->ordered_length;
}
and that should do it.