Without a jig, and assuming that you are using
the type of cove that has a flat back (also called Crown Moulding
in America) there is another technique.
You will though require a mitre saw that can adjust the bevel
angle (the angle of the blade from vertical when looked at from the front) as well as the mitre angle(the angle of the blade when looked at from above).
Set the saw mitre angle and the bevel angle as per the calculator below.
Lay the Cove flat on its back on the saw table.
Make the cut.
The maths to work out the mitre and bevel angle is rather
complex and so the calculator below will help.