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When making custom lamp sizes is not too hard to calculate the panel sizes. 
You need to remember the value for pi (3.1417)

Start with the bottom diameter you want.  Multiply it by pi.  Divide this
distance by the number of panels required for the lamp.  This gives the
size of the bottom of the panel. 

Do the same for the top, but make one more calculation.  As the top has
to fit into the size of the vase cap, you need to take account of the thickness
of the glass.  So, do the calculation as for the lower edge of the panel, and then subtract the thickness of the glass from that width.  For most glassthis will be 3mm (ca. 1/8" or .125").

Set up a horizontal line the calculated width of the bottom of the panel. Divide it and draw a vertical from the centre of the line.  This line should be as long as the panel you are making.  At the top of the vertical line draw another horizontal.  Measure off one half the calculated top distance on each side of the vertical line.  Join the points on the lower and upper horizontals to give the shape of the panel.

Worked example:

Bottom diameter: 200mm (8")
Top diameter: 50mm (2")
Panels: 8
Pi: 3.1417
Glass thickness: 3mm (1/8"/ 0.125")
Formula for bottom: dia. x pi = circumference / no. of panels = width of panel
Bottom diameter: 200 x 3.1417 = 628mm/8 = 79mm
(Bottom diameter: 8" x 3.1417 = 25.13"/8 = 3.14")
Formula for top: dia. x  pi = circumference / no. of panels = uncorrected width - thickness of glass =width of panel
Top diameter: 50 x 3.1417 = 157mm / 8 = 20mm - 3mm = 17mm
(Top diameter: 2" x 3.1417 = 6.28" / 8 = 0.79" - .125" = .66")

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