Custom Shipping weight/dimensions Calculations

We have a product in our store that is sold by the pound. I currently have it set up as a variable product, with a single variation of 1lb. Customers can then select the quantity by choosing the number of 1lb products. I’m not sure if this is the best way to have it set up, but it’s what I’ve figured out so far.

In regards to shipping, this might be a little more complicated. We ship these items in buckets. The buckets weight 2.5lb and can hold up 30lb of product. So if someone were to buy 20lb of product the shipping weight would be 22.5lb. If someone bought 35lb of product, the shipping weight would be 40lb. Is there any way to add a custom weight/dimension calculation to a product that could account for this sort of thing? Maybe via JavaScript?

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