Pete,

It would help if you attach or send your program. I have included something, but I don't have Tutor to try this out (and haven't touched it in about 7 years). If it doesn't work we'll have to find the vertex of the cone, then the distance from it to the intersection point between the offset plane and the axis of the cone and finally use "circle_by_cone".

program PETE[WM1,WM2]
element_array MEMORY[100]
element PLN,CON,CIR
coord PT
vector DIR

! measure cone CON
mcone(CON,10)

! measure plane PLN with 5 points
mpl(PLN,5)

! offset the plane PLN
PT=PLN
DIR={PLN|CX,PLN|CY,PLN|CZ}
! if your offset is 50
PT=PT+50*DIR
! or PT=PT-50*DIR depending on the plane orientation
PLN=PT

! try now this, not sure if it will work
inter(CIR,CON,PLN)
! DM of the intersection circle CIR is your number