I made a program a little while back for TI-83 and 84 calculators to calculate the size of a shroud template based on a provided large diameter, small diameter, and length (calculations from Model Rocket Design and Construction)
Program: Shrouds
:Input "SMALL DIA. ",E
:Input "LARGE DIA. ",D
:Input "LENGTH ",L
D-E)/2=Y
:sin-1((1/squarerootof(((L/Y)^2+1))=A
isp "ANGLE IS: ",360sin(A
isp "SMALL RAD: ",(E/(2sin(A))
isp "LARGE RAD: ",(D/(2sin(A))
If you don't have a programmable calculator, the calculations should still be fairly easy to use with a scientific calculator, just do the above one step at a time. The output is the small radius size, large radius size, and angle of the template (starting from a fixed point, see attached image).
Oh, and make sure the trig calculations are done in degrees; and since you just want to make nose cones, the small rad should always be zero.
View attachment shroud.PNG