On position sizing and stop loss placement

Hey everyone, still trying to dial in my risk management. I've been doing okay identifying setups, but my issue is often around position sizing and where to place my stop loss. I try to scale in sometimes, but then if the first entry goes south, it throws off my whole sizing for the second. Is there a general rule of thumb people follow for what percentage of their allocated risk should be on the initial entry vs. a potential second entry, assuming the market confirms your initial bias?