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I also found out how I can make the stitches more even. Instead of drawing the small auxilary lines, I drew a dot on the main line were each interlacing stitch should be. Then, I counted the linen threads, starting from the dots. Like this:
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And a picture of my "goal" to keep me going: a 14th century whitework border in interlacing stitch:
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This picture is from Pesel, L. , Newberry, N. (1921), A book of old embroidery, with articles by A. F. Kendrick. London: Geoffrey Holme, “The Studio”