Stuart,
With the thickness of the interior strips you've selected, it really isn't possible to make a diamond pattern fit in segments that are only 1-1/2" tall when the repeating unit has to be 3-3/4" wide. If making the feature ring taller isn't an option, you might consider this -
Keep the diameters of your bowl exactly as they are, but change the number of segments from 12 to 16. This will reduce the Segment Edge Length from 3-3/4" to 2-13/16" which will make it easier to fit a diamond pattern. Your Width of Cut should be 1-1/2" (or 1-7/16" to be precise). You'll have to make slight changes in the strip widths to make it fit, though. Reduce the width of the Holly strips to 1/16" and the Bloodwood from 1/4" to 1/8" and keep everything else as it is.
The only other option I can think of is to leave your design exactly as it is, but increase the Width of Cut to 2-1/8". This will give you a Segment Edge Length of 4-1/4", but if you trim 1/4" off the inside edge of both strips, it will bring the Segment Edge Length down to 3-3/4" and the diamond will nearly fit. To be sure, I would still reduce the strip width of the Bloodwood by 1/8".
Finally, if you plan on separator strips above and below the feature ring, take 1/16" off both of them and increase the height of the feature ring by 1/8". The shortness (and wideness) of that ring is really the issue here.
The attachment shows a 12 segment ring as you sent it to me and another ring with 16 segments with the changes I suggested.
Lloyd
