Plant based compounds in hydrocarbon polymer materials

Hi there - I am modelling the production of a polyurethane product, where the raw materials (the isocyanate and polyol polymer materials) have around 50% bio-based composition (i.e. they are a mix of hydrocarbon and bio-based polymers, developed as a lower carbon product).
What is the approach please to modelling of polymer production from biobased feed stock, as compared to fossil derived polymers? Also, how does this affect the biogenic carbon calcs within the model?
Many Thanks, Kris