First post here: Question on position sizing with partial closes

Hey everyone, just joined. Been trading equities for a bit, trying to get my head around futures more consistently. I've been experimenting with taking partial profits, say 50% of the position off at a first target, then letting the rest run with a trailing stop. What I'm struggling to get my head around is how you guys typically size your initial position when you plan on taking partials. Do you size for the full initial risk on the whole position, or do you account for the expected reduction in exposure after the first partial close? Seems like there's a few ways to slice it, and I want to make sure my risk management isn't off. How do you factor that into your R-multiple calculations or max risk per trade?