HI,

The pattern piece is a giant slice of pie with radius P units and an angle of D degrees.

Suppose the cone you want has base radius of R units and height H units, in your case that's R = 1.25 inches and H = 48 inches. Formulas for calculating D and P are contained in a response Stephen sent to a previous question.

Here's the "general" solution for any similar cone. Let's say you know the height H and the opening you want has a radius of R.

Let P = the radius of the circle. Then P = √(R² + H²).
Let D = the number of degrees of the circle to keep (as a pie slice). Then D = 360(R/P).

Penny