No no no no no.......
The seatpost CLAMP you are using is 25.4mm inside diameter Beelzebub because the outside diameter of the seatpost TUBE is 25.4mm
You need to get this right as Newschool bikes and Burners run 25.4mm outside diameter seatposts. If you buy one that size it will be too big.
Problem is I don't know off hand what the actual OD of an oldschool post is - I think it's 22.2mm or something like that, but someone on here will know. Don't forget the thickness of the frame tubes have to be removed from the 25.4mm ID of the clamp to work out the OD of the tube....
Fook, I've confused even myself and I still don't know what the shim size should be, but it was also me that suggested to Deeman cutting a coke can to make a shim, so give that a go first