Very accurately measure the wall thickness. Multiply times 2 and add that to 27.2.
Odd thing though, I was unaware that any Cannondales of that era used a shim to achieve 27.2.
FWIW, the best seatube shims I have found for 27.2 are the Thudbuster manufactured parts. Far better quality than Problem Solvers brand on account of being longer and anodized.
PK