@TurtleCove I haven't seen such an app but if you can explain more about what exactly you need i am sure i can help you with that.
I am a developer myself and working on an app i am sure this can be achieve easily with the help of customizing your theme or if you want customer to checkout with that amount Store Front API can be used.
Think of it as though I sell fabric.
I sell this fabric, by the square foot.
The shopper will decide which fabric they want, from my store, and then they can select from a dropdown, the width and the length they want.
Based on their selection, my storefront will show them the cost.
Ex: They pick 6 inches wide, and they pick 36 inches long, for the fabric that is $4.25 per square foot.
Shopify would perform: 6 x 36 / 144 x 4.25, and then show the price.
you can use the variant as the width of item if you have multiple width of the same fabric like you have cotton in two widths 6" and 8".
secondly if you want the user to order a custom length then a textbox would suffice the need, where a user can enter a desired amount.
the user defined amount can be stored in meta-fields of the orders.
Here's an example: https://pebbletilemosaics.com/products/natural-earth-pebble-tile
Here's a Fabric example w/ fractional length: https://stylemakerfabrics.com/products/seaside-floral-digital-print-linen-shirting-coral-navy
Reach out if you would like to have us help you. https://www.fdgweb.com