Hi ffsear,
For me, this would be governed by the strategy employed and the instrument(s) being traded. Assuming that the focus is on one instrument at a time, I would limit my leverage to 4:1. So, for example, in the case of the Dax, my maximum exposure would be £20.00 per point. A simpler way of looking at it is to trade at £1.00 per point for every £2,500.00 in the account.
As to why: using too much leverage is right up there with the reasons why so many traders blow up. For me, 4:1 is enough to make it worthwhile - without being so high that I need to wear nappies.
Tim.
On this line I would only adapt my £ risk according to the total account capital - if I had made £50k into £100k say, I would possibly drop the risk % to 1%. But whatever is the max risk per trade is also the min risk per trade, no second-guessing which might be the winner and which is "more risky".
My 'ordinary max' on those kind of numbers, assuming current margin levels would still apply, would be e.g., around 40ish pp on ftse100, leaving a little spare in case something else pops up elsewhere. But there's no way I'd be thinking of 2-300pp.