Layout of a truncated cone

[Catnip]

Im confused...

radius should be half the diameter so...
r=1.25"
R=2"
or am I misunderstanding something?

If you were making a 2.5" cylinder and a 4" cylinder, you'd be correct, but when you're doing a truncated cone unfolded, the length of the cone and difference in diameter, changes the arc radius of the arc length that forms the circumference once the shape is rolled into the truncated cone.