To my mind, it would be better to calculate risk according to the average daily profit. The idea is that maximum daily loss should not exceed the half of average daily profit. Then you can divide this amount into the number of position you may have during the day in accordance with your trading system. For example, if your average daily profit is $100, your maximum loss per day should be $50. If the average number of trades per day is up to 5, then your risk per trade should be $10. This is very simple calculation without taking fees into account, but if the idea itself seems interesting and useful, you can improve it depending on your particular trading style. Such approach to risk management helps trader to avoid psychological pressure caused by huge daily loss, because it would be quite easy to recover after maximum daily loss - it would take only one day. Newbie traders that have no track of records should just start with the lowest position sizr available and then make those calculations based on their performance for a few weeks. The method described above is widely used among active daytraders. It could be also used for other trading styles just with minor changes and amendments.
If you have accumulated enough statistics on win:loss and expectancy on your trading strategy, use fractional (~1/4 - 1/2) Kelly.
Noob here, until now daytrading with a small U.S. account. Here is my perspective. With a $10k account my max position size was $2k. My max RISK is the amount I stand to lose if the stock turns against me, and my stop is usually fairly tight, sometimes less than 1% and almost never more than 2% down. Depending on where I see support. I don't think my stop has ever been as much as 3% down from my order price and I never use market orders. My preset is 3% down and I always end up moving it up to just under a presumed support level, which if it wasn't there I wouldn't be in the trade. At a 1% stop, my risk is 1% of my position, and if my max position size is $2k, then my max risk for that position is $20. My percentage of risk to my entire account from that position is then 0.2% and if I have ALL my money in play, which I have never done so far, then my max risk is about 1% of my account. If my average stop loss was looser, around 2% which is probably closer to most traders' average stop, then my max risk, with all money in play but no margin, would be 2%. I wouldn't mind a 2% risk to my account total, as long as my potential profit were at least 5%, but I have never played it that loose. Of course setting a stop is no guarantee that your loss can't exceed that. Trading could be halted and the stock gap down, for instance. Or you mess up and buy big into a very illiquid stock and price slides heavily while your stop order is being executed. But it's what I go by, anyway. Initially my plan was to use a max of 2% risk, but I have never been even close to that. Now that I am funded properly and not under PDT restriction, I COULD run bigger positions and several at once, but for now I will not, though maybe under some circumstances I could buy 100 of a more expensive stock than the $20-and-unders I have been trading. But basically my risk is still going to be almost comically small for the present. I am not going by a particular percentage because any realistic or commonly used percentage will be considerably more than the situation will bring. Ultimately, I hope to reach a point where I am comfortable having all my money in play at once. Then my max risk figure will have some relevance, and it will probably be 2%. And no position will ever be more than 1/3 my total account, no matter how good it looks. So max risk to my account from one position will still be less than 1%. I know that sounds pretty tight, but I guess that's my style, and it kept my account together while I was just starting out.