Pondering Smut Setups

My current WIP has me contemplating what is the right amount of lead up/plot for low plot smut. I feel there is some required setup, so I can't quite go full PWP. But I'm not entirely sure how to keep enough tension through the setup.

I usually read some similar stories, and/or read up on specific techniques, but I haven't figured out how to categorize this yet. Usually, the hardest part of writing smut for me is the actual smut, so this is an amusing issue to be having.