I want to risk 300 USD on a system that generates a maximum drawdown of 7%
Is this the right calculation (EUR/USD)
300/7 = 42,86 USD per percentage point
42,86 USD * 100 = 4286 USD total size of trade
4286 USD / 100.000 (std lot) = 0.04 lots
There are some great calculators at the Tools section, you can check your calculations over there 
Hi there, I have observed that many brokers have an add-on packages , where the Expert Advisors ( I have an example in my mind - The Mini Terminal EA) could make that type of calculations automatically instead of you.
It may be, it may not - but any bet which goes 40 cents a pip on an account balance of $300 sounds risky to me ! What is your stoploss distance in dollars ?