Martingale Trading And Probabilities

I've recently started trading again. I traded, with some degree of success, a few years ago (forex), and have decided to use the same strategy. Broadly speaking I trade the news (inasmuch as this is possible with the company I use) on fairly standard FX pairs (predominantly EUR/GBP) and the FTSE 100 (may expand my pool of interest to include some UK stocks); I'm dabbling in bitcoin trading too (merely because it may provide the volatility needed for my strategy).

Anyway, the above is probably fairly standard and fairly dull to you. But my loss recovery system is Martingale (aka doubling down). So, on my current trading pot of £500 I'm looking for a 20 point move at £1 a point generally (some things like bitcoin which attract larger spreads I'm doing say 100 points at £0.20 per point) for both sl and limit.

Now, obviously martingale is only a loss recovery system, it doesn't, of itself, secure profits in the medium term. The quality of the trades is obviously important, as someone who flips a coin to trade will probably be right 50/50 and thus may expect to encounter a bank breaking run every so many trades. My question is, how do I work out my likely "run of luck" based on different percentages of correct trades?

TL;DR- Assuming a trader is exposed to a £20 loss or a £20 gain on his initial trade (i.e. his limit and s/l- putting slippage aside for a moment), and can lose 4 times in a row what is the probability he will bust out in 100 trades if he gets 55% of his trades right (and how does this compare to the probabilities for a trader who is right 50% of the time).

I'd like to make a 30% return in the next few days, and then scale it up. I reckon, being realistic, I get the market right about 50% of the time. How much of an advantage would it confer for me to get it right 55% of the time.

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