How does one fill out form 2106 if your employer covers some cost but not all?

Example...

Drive from home to customer and back = 100 miles

Employer only pays .40 for mileage for anything above what you would drive roundtrip to office daily = 78 miles

employee can claim 22 miles @ .40

according to IRS can't employee claim all 100 miles @ .50 since he didnt go to office?

if so how do you fill out when employer does cover some?

Full year Example:

Total Miles on Car = 31286.5
% for Business = 78.30%
Miles Claimed Per Employer = 10904 @ $0.40
Mile Not Claimed = 13592
Total Business Miles = 24496

Thanks!