He asked for a 100mm radius flare on a 180mm diameter port. I'll use his request to illustrate the maths

Assume the port is to be made from 5mm thick PVC tubing with an inside diameter of 180mm and an outside diameter of 190mm

Cross section of a large custom flare

Decide on the thickness of MDF or other timber you are making the rings from. In this case it was 18mm

Step2 - Make a drawing like that shown above

Work out how many rings are needed. Here we need 7 rings to produce a 100mm flare. The last ring is to give enough surface area to bond to the PVC tube. Once the bodyfiller is added, a flare of this size will weigh a fair bit!

Mark the drawing with the distance each ring is from the outside face of the flare as shown above in blue

Step3 - Work out the inside diameter of each ring

For each ring, work out dimension "a" as follows:

As an example, the green triangle is used to work out the value for ring2.

c = radius of flare

c = 100mm

b = 100 - 18

b = 82

From pythagorus' theorem

c^2 = a^2 + b^2

re-arranging terms gives:

a = sqr(c^2 - b^2)

a = sqr(100^2 - 82^2)

a = sqr(10000-6724)

a = sqr(3276)

a = 57

The inside diameter of each ring is dimension "e" minus twice the value for "a"

As you can see from the drawing, dimension "e" equals the inside diameter of the port plus twice the flare radius, in this example 380mm.

Working all the rings out with the same method gives:

Step4 - Work out the outside diameter of each ring

When gluing the rings together, I like an overlap of 30mm. This is achieved by making the OD of each ring equal to the ID of the next larger ring plus 60mm.

The outermost ring has to double as a mounting flange, so it should have an extra 30mm all round in addition to the overlap.

In this example, the two smallest rings can be the same size.

Putting all this in a table gives:

All that remains is to cut the rings and glue together. Once they are dry, glue the PVC into position. Referring to the drawing, we can see that for this example, the PVC tube is positioned so that 10mm of ring6 is visible from outside the port.