The inside leg measurement isn't the final measurement for the seatpost IME.
I used to have my cleats set up very far forward as I prefer to pedal toe down. Some people prefer to pedal heel down.
Over the last couple of years I've moved them back after watching a series of videos. This obviously has an impact on the overall height and I've found my old seat post sleeve made the seat too high. As it happened,
@Tenkaykev very generoulsy sent me several as his seatpost is perfect without them.
Also, I recently snapped one of my saddle rails, but was using a colour Brompton saddle I bought very shortly after getting the bike. I replaced it with the orginal black version and, again, my seat was too high as the saddle hadn't worn in to the same degree.
I have a 32" inside leg and have the extended rather than the telescopic post and it's not at the top. A lot will depend on how you pedal and which seat you have. It will depend on how your pentaclip is set up.
I'd avoid the extended one if possible as it's just more weight to lug about if you don't need it.
That said, when I hired a Brompton, it came with the telescopic post and I found it useful. You can set the height perfectly by pulling the bottom section up fully and then setting the top section to the height that works for you. When folding it down, you only collapse the bottom section and leave the top locked. this way it's always the perfect height, without the need for the seat post insert.
I also found that even leaving the top section locked in place, I ended up with a smaller folded package than when I switched to my own bike with the extended post.