Its really no tthat hard and you are over complicating it. the 70/30 split or whatever will be reflected in the rate of duty applied tothe fuel.
If it were a 70% full duty 30% rebated duty split, the amount fixed by HMRC for red diesel for leisure boaters would be (0.57x0.7)+(0.10x0.3) or £0.43 duty per litre.