A question on the other forum lead to a little experiment to explore some difficulties in hemming "pocket rounds", horrible things that are best forgotten. Fashionistas apparently wear them instead of a handkerchief in the breast pocket.
These beauties have usually diameters of 11 or 12 in., about 300 mm, so I chose that size. The hem is 3 mm and folded in is 2 mm. The piece of scrap fabric in the photographs is a bit thicker than would normally be used, but is supple enough. As it is not to be a real "pocket round", only a quarter is used, enough to tacke the problems.
When completed, line B will have assumed position B' and line C will be under the hem at position C'. That means that the maximum amount of fullness that will have to be eased in is on line B, as it will end up nearest to the centre of the circle.
On or near this line a row of gathering stitches is made. As it is near the edge, if possible use a straight stitching foot and needle plate.
The gathering stitches are put under tension. Because of the narrow hem, only about 40 mm has to be fulled in over the whole circumference, wider hems require more fullness processing. The hem already starts to form itself.
The hem is pressed in.
A pressing gauge can be handy, just a piece of thin card or pattern paper extending to the finished hem line.
Two ways are open to proceed now: one can roll under part B-C and fell by hand, making a roll hem that can even be smaller, or one can machine stitch the hem and fold under the allowance at the same time. I chose the latter. It is also possible and quite justifiable to baste the hem with smooth, thin thread first, press, and remove the temporary stitches.
The result shows that the stitching could have been a bit neater, but the light was so bad I could hardly see what I was doing. The important thing however is: no puckers and no distortion of the fabric (note the faint striping), which was the subject of the original question.
N.B. The lighting in the pictures is not up to scratch; I borrowed this camera and am not at all familiar with it.