70 is too small a sample size ans saying 30% is too simplified. what if I chose 70 races where the favourite was $1.50 or less. I would expect at least 50% strike rate not 30%. What if I chose 70 races randomly but they all had favs at $4+ prices. In that case the strike rate should be 25% or less.

The better option is to determine how many winners you should have got from those races. Its simple enough to work out . 1/odds = chance of winning. Add up all of the favs using this formula and it tells you how many winners in those 70 races were expected. This doesn't account for commissions so you should adjust for those too if you using tab prices. For betfair you don't have to worry as commission comes out after the winning bet so the odds on display are generally to 100%.