I agree with you. I've always made the base taunt and the top cloth material fuller.This allows the fullness of the pleat to be seen on the top side.

Hey Warren!
That was actually my video. I'm not sure what you mean by bunching when sliding across the seat.
The reason for the added 3/8 is because the base material controls the finish width of the pleats. I haven't found a way to prevent this because no mater what once you fill it with cotton the base rounds out a little causing the pleat width to shrink as well as the entire section. I haven't had any problems with bunching.

Id love to hear how others have approached cotton stuffed tuck and roll. I know there are multiple ways its done. This is just the way I have figured out how to do it.

I would agree the base calico/muslim should be the finish size or taunt.

the calico/muslim controls the width and the structure of the tuck and roll, if you have 8mm extra on each pleat (2 inch) on the bottom there's nothing to governing the size of the flute apart from the stuffing? i think in a very short amount of time the flutes may look poor due to stretch?

Thanks for the replys. John no offense or disrespect I love your channel. You do great work on it. And I agree there are many ways and techniques. I just think the bottom should be taunt, especially a one piece bench seat cover. I like tight fitting cover. I don't see where the Muslim would shrink. It may look like it did but if you pull it tight from side to side, you should get the length you had when you start. That being said for stuffing pleates. I do allow for shrinkage on top stitch pleating when doing todays style with sew foam. I allow 1/8"per pleate.

I wasn't offend at all. Sorry if it seemed that way. Glad you like the channel!
Next time Im doing some stuffed tuck and roll. Ill do some experiments and see if I can get it to work the way your describing.

What’s your channel John? I’d like to check it out.

https://www.youtube.com/watch?v=re4ybTju-t4

I probably going to have to use your idea to make my piping tins. Can't find anyone that has a 3in pair. Ordered a pair from J&L Fabrics and they sent me 4". Called C S Osborne and they don't have any and their maker went out of business.

I just fold over the end of the stuffing, wrap some thin plastic over it and use a long ruler to feed it down the channel.A T shaped piece of soft top window helps guide the cotton into the channel.

I pre make the cotton inserts to the desired size and wrap them with a thin (1/16") headliner smooth side out. Slide them into the pleat channel with a ruler or cut to size waterproof panel. Pre making helps to keep the cotton consistent and eliminates bunching.