“[The] value of the car does not bear on the use of overheads. What [VWFS] says is [that] if [that is done] it would be 80%, but 50% is fair. But why? The appellant does not say. 50% is an arbitrary selection of a figure. No analysis has been put forward. [This] comes from the weighting exercise. HP contracts [are] treated as two transactions. [It is] wholly unexplained as [to] why it is fair to treat HP [transactions] 1:1. Why not another fraction?”