Change shipment depending (also) on length from custom input field

[tonsenson]

Hey again,

same shop as in my other post, here we go again:

Joomla 3.4.1 with VM 3.0.9
While I fixed a problem with the weight in another post, I can not solve this problem:

Customers are able to choose the product length by themselves. So there is no fixed length value; the products are saved in the backend without any length value.
I tried (again) the free version of Shipping by Rules plugin to respond to the changing value of length. I found the array key [product_length], in which I save the width values of the single products and it is all working and stuff, but still I do not get the right shipment method.

I tested the plugin by setting a fixed width value for one product (in the backend): VM displays the desired shipping method.

I assume that the shipment method is checking directly via the database, how long the products are and then saves the possible methods and displays them on the page.
In this case, other than in my other topic, I really can not set fixed width values for a product or any variations, so I need to fix that by a little hack or something.

Any ideas on that? Thank you VERY much in advance,

Tonsenson