I wouldn't necessarily say there is a "metric" I use for buying or selling a stock or ETF.
For one, on the selling front, the simplest way to develop an easy method for selling is to simply develop one for buying. This sounds confusing, but it's really not. If you develop a proper investment thesis and understand why you bought a stock or fund in the first place, it becomes very easy to sell it. You sell it once your thesis is no longer true. And no, a thesis can't really be based on price movement. Price movements over the short term on the markets are simply unpredictable. It can take years for an actual well developed thesis to play out.
In terms of buying, I look to find companies with high returns on capital, returns on equity, and attractive price to free cash flow ratios. Growing earnings and growing revenue is a must.
Even if all these metrics look optimal to me, it still takes an extensive amount of time to research the company. Its competitive position, moat, margins, outlook, management team etc.
After everything checks out, I'll make a purchase. And from there, I'm well aware of what I need the company to do fundamentally and if it doesn't, it leads me back to my first situation where I know why I bought, so selling becomes easy.
In terms of the RRSP/TFSA, I'll typically hold my US stocks inside my RRSP and my Canadian dividend payers inside of my TFSA. This allows me personally to mitigate and defer as much taxes as I can but keep in mind my position is going to be much different than someone elses. Taxes are very personal