You probably need to define whether you're measuring P&L against SP, or some other mechanism. For this type of thread, SP is usually least contentious measure, as it's easily identifiable, and least open to challenge that 'your' price was never available/only laid to ten-pence.
Best of luck with it.
Given one firm was offering 4 places on the race I'd suggest he just do write ups and not advise bets.
Agreed. Measuring to a Level Stake is the way to track it.