Thanks for the answers, but I've just realised I asked the wrong question!!
What I meant to ask was not about adding lots to a position you alreay have,
but about how many contracts you buy per trade when you open a position!
The chap in the article based how many contracts he buys on his performance so far.
Say his account size was $10000, if he traded his account up to $11000
(+$1000), then on his next trade he is allowed to buy 2 contracts,
then he must make $1000 on his next trade on each of those contracts (another $2000)
to be allowed to buy 3...and so on.
Sorry, could you tell me your thoughts on when you allow yourself to buy more contracts?