Calculating the top and bottom widths
When doing 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, subtract the thickness of the glass time 2 (the glass thickness is on both sides of the circle). Do the calculation as for the lower edge of the panel, and then subtract the thickness of the glass from that width. For most glass this will be 3mm.
Bottom diameter: 200mm
Top diameter: 50mm
Glass thickness: 3mm
Formula for bottom: dia. * pi = circumference / no. of panels = width of panel
Bottom diameter: 200*3.1417 = 628mm/8 = 79mm
Formula for top: dia. - glass thickness *2 * pi = circumference / no. of panels = uncorrected width – thickness of glass =width of panel
Top diameter: 50-6= 44 * 3.1417 = 138mm / 8 = 17mm
Determine the shape of the panel
When you have determined the widths of the top and bottom of the panel, you are ready to draw up the shape of the panel. 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. This is determined by the method outlined in the Panel Length tip. 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.