You can tell many different stories with the same set of stats. From your numbers...
18 days on a + 0.3343 trend might say you're just suffering a bit of drawdown and should be on £6
Likewise after 18 days you start to get the first hint at the normal distribution of your daily PL and you can start to infer things from that...i just did it freehand and it obviously won't go negative like my red line did.
Then again, even with a random strategy that's 50-50 up/down per race, hitting 79 or better out of 154 is a 40.5% chance, ie very likey. 90 +ves out of 154 would be 2.18% and much more conclusive. Make it 100+/154 and that's a nailed on strategy because you'd only hit that 0.01% (actually 1 in 7,651) of the time if you only had something random. (all assuming avg win is approx avg loss, there are of course strategies that win 500 out of 510 races and lose money)
Just a reminder : if you're doing this day after day with tiny stakes and/or just staying connected for hours on end there's a good chance you'll end up with a restricted account. And fair enough too, some of us are trying to work and we don't want Betfair's servers overloaded with data harvesters all funded by our commission