"but also risking hiding that fact in my backtest" . What I'm reading into this is that you assume that you will trade out at a stop level during a gap in your backtest, when of course that's not always possible. Do you have the ability to modify your backtest so that this assumption is challenged? Personally I'd assume I always traded at my limit when I was placing resting orders, but that I'd trade somewhere between my price at the worst point of a bar for stop orders (or if I don't have bar data, at the subsequent closing price; i.e. I lag everything by one time period to get my expected fill price). You know reality is somewhere between these two places, and you can get an idea of how lucky you need to be in those gaps. You can also compare actual trading to backtest to see how often you actually trade out near stop levels.
This is what I would do if the idea would show a potential. Real question here is why would I spend time on more accurate modeling of those stops if initial "best case scenario" test for them showed consistent degradation of the values I optimize for across most of my test periods?
I think you said earlier that your average holding period was a few days. Putting this together, I feel like 10% might be too large: is 10% the notional value of your exposure (i.e. with $100K you buy a $10K worth of stock) or the amount you expect to lose before the trade is closed? If the former you are probably okay, if the latter I'd be concerned. The missing piece of information (forgive me if it was earlier in the thread) is how many stocks you typically have in your portfolio, and are you using any leverage?
This is a very good question. Someone can probably write a book to answer this.
How would you define value at risk for any stock? I can try to calculate it according to your definition.
Here are my thoughts.
The reason I keep mentioning 10% is - I'd size
up to 10% as a general rule. For example 2 of my high turnover MR strategies have a cap of 7% currently, all shorts have this cap, so strategies would be as low as 5%. Pedantically speaking this IS value at risk and more for shorts, so is any position any stock trader in the world is holding no matter what they think is at risk, as this is just a nature of stocks. But due to high recovery factor of my strategies with a lots of trades I don't believe it is optimal to size to the absolutely worst case scenario. Nevertheless I do pay a lot of attention to worst cases, study every single biggest DD/crash/volatility spike over last 30-70 years and try to avoid too much of exposure to likely to become highly correlated trades when that happens.
Depending on the definition of the "instrument" you might say I trade one as stocks will have high correlation with the market especially with high vol. But I diversify by a type of strategy / hold periods / long+short and exit types. So, perhaps, something similar to "
instrument diversification multiplier" you mention in your blog can be applied here.
My average exposure across all strategies was ~75% from Jan to May. Available leverage is 2x (t-reg margin account). Occasionally I'll get close to max but it rarely happens due to too high number of positions. Predominately due special margin requirements, when basically 1 position needs 2-8x amount of capital to hold comparing with normal. I'm contemplating to start skipping those but don't have sufficient stats yet to make this decision. Avg positions at any time is probably around 8-10.
Attached couple of graphs with long/short exposure per strategy and total portfolio positions