The pierrot jacket was common during the 1780's, exactly where I try to focus my 18th century wardrobe. It's a short jacket with a peplum in the back, but there are some without peplums as well.
and there are some where there is a visible waist seam and the peplum is pleated into that.
When I asked around for advice I got the tip from Åsa to make a compromise, and have the back piece go into the pepluem, but attach the rest of it at the side seam. She has done that in her 18th century caraco. Later Johanna also showed me her jacket constructed in the same way.
So with that done I got my basice bodice pattern that I made in June and cut out a muslin. I was happy with the back piece, then it was time to cut out the rest of the peplum. On this it was mostly a case of guess work, looking at the kind pattern pieces that Åsa had, and what they look like in Patterns of Fashion, but basically just cutting an elongated piece of fabric and pleating it in different ways until I was happy. I cut the length and shape of the side of the peplum when I was happy with the pleats.
Ignore the bad fit of the muslin. There is no chance that I can get my stays onto my dress form, so it's quite useless for 18th century fitting, but it's still good when I want to attach skirts and the like.