In direct answer to your question, it would depend upon what the Contractor pays for and whether private mileage is reimbused or not by the Contractor. If it is then this would be classed as a P11D benefit to the employee, but the cost would be included in any calculation.
The key is at paragraph 11 of the SCC which states;
' ... amounts paid by the Contractor ... according to the time worked on the contract.' (NEC4)
How much does it cost for the Contractor to employ that person, for the time thay are working on the contract, as defined by section 1 of the Schedule of Cost Components?
To be strictly correct you would look at each individual and their pattern of behaviour to determine what is realistic. That is very time consuming, however, and also subject to change over time as personal circumstances alter. Consequently it would be more sensible to apply a 'general allowance' based on a sensible average to calculate the number of miles per year for business use, where private use is not reimbursed.
This is the same principle that the daily or hourly rate is based upon as you apply an adjusted rate (taking account of an assumed assessment of non-working time) to the time worked on the contract.