Before sizing a trade, what single number do you calculate first, and why?

Before I think about position size, I want to know the invalidation level. The distance from entry to the point where the idea is clearly wrong sets the per-unit loss, and that loss times the number of units is the real risk per idea. Many people size by how much they want to make or by a round number of shares, then place a stop afterward. That order reverses the logic. Once the invalidation distance is fixed, I check the dollar amount I am willing to lose on this idea against the daily loss limit and against any other open positions that move with it. Two ideas that share a driver can quietly double one exposure, even if each looks small on its own. I am curious how others sequence this. Some people start from a fixed percentage of equity, others from a volatility measure, and some from a maximum dollar loss they set in advance. Which one comes first in your process, and what made you settle on it?