I have found 4 years of data to be ideal.

What I found was the SR was consistant at 25% accross all the systems batched together, but the average price started to drop away.

So I then started to bet the horse only once instead of multiple times because it was selected by multiple systems & the POT improved.

If one is using multiple systems .
Try not to double up on the bet because it is selected by multiple systems e.g. If the same horse shows up 5 times, it does not mean it has 5 times the chance of winning , so its best to only bet it once,, for better long term results.

Cheers.

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Cheers.