Accucraft GS 4 Chimney&Petticoat

HI,

After carrying out some of the modifications recommended for the alcohol-fired version of Accucraft's GS4 (incl. Gordon Watson's combination levers and nozzles and new wick material) I am still not REALLY happy. The performance is quite OK but could be better.
So I referred to some textbooks and articles on the subject, e.g. Martins Evans' books and J.v. Riemsdijk's articles in the G1MRA Newsletter.
As regards the front end, i.e. arrangement of nozzle, petticoat (chimney extension) and chimney, both authors recommend the same geometrical dimensions. I built two templates to check these dimensions on my Accucraft GS4, see first picture
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The template tapered 1:6 governs the height of the chimney top above the nozzle, which appears to be OK, see second picture
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However, after applying the second template tapered 1:3 which determines the extension of the petticoat, it appears that the petticoat is at least 1/4" too long!
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How's this? This template has 8 stripes green/yellow each 7/64" wide. The template shall sit in the blast nozzle with the tapered part still inside the chimney extension or petticoat by about 3/16". So at least six of the eight stripes should show underneath the petticoat, however, on my GS 4 only three and a half are not covered by the petticoat. That means it is too long and most likely impedes draft!
Has anyone else observed this?
Has anyone unscrewed the petticoat? How is it fixed? I would like to shorten it to make another test.

Michael