These comments are more general, but can be applied to this problem:
- A trading strategy typically consists of the following items:
Alpha Model: this model defines the edge of a trade; it can have input information that includes market data, signals, etc. the output should be a theoretical price for the asset you are trading. We expect the asset price to converge towards the theoretical price.
Risk Model: this model dictates trade size/inventory management. For example, if an asset's current price is $1 and our theoretical price is $2, we might buy one unit with the expectation of making ($2-$1)=$1. However, if the price drops to $0.90, we now are faced with the following dilemma: if we are 100% confident in our theoretical price of $2, we now see $2-$0.90 = $1.10 in edge. However, we are currently down $0.10. The risk model should take this input and determine whether we should put on another unit of position, or possibly sell out some risk.
Execution Model: this model determines how an opportunity to add or remove inventory is actually executed in the market. It should account for execution cost (spread crossing/slippage) as well as market impact (less important for small order sizes relative to net liquidity).
As this relates to your question:
- It seems there is little separation between risk management and alpha. First determine what your alpha is, then worry about capturing as much of it as possible, while respecting risk limits. The question of when to scale out of position is really a question of whether there is still alpha in the trade. Here's an example:
Time 0: You see an asset at $1 and have a theo price of $2. You buy the asset at $1.
Time 1: The asset has moved to $1.50 and your alpha model predicts a theo price of $1.75. That is, you have made $0.50 of PL (unrealized) and have $0.25 of edge left in the trade.
Time 2: The asset has moved further to $1.60 but theo has moved to $1.60 as well. You have PL of $0.60 and remaining edge of $0. Therefore, you would sell out this inventory since it no longer has edge.
This might be a different way of looking at trading, but it is far closer to a professional type of management. Think about assets and theoretical prices, not dollar amounts because that is one level more abstract than the asset price. I think the most common example of this idea is a moving average strategy (not saying there is any alpha here, but this is the idea behind it). If one thinks that price will converge to some MA, then if price is below MA we would buy and if above we sell. This is simply a bet of convergence to theo, which in this example is the MA.