First, thanks for creating Dorico.
I’m a German Dorico user. Mostly I use DIN A4 (called “A4” in the dropdown menu for paper format/page size). In Layout options Dorico shows 209.55 mm x 296.69 mm in the settings for height and width and the little black box to the right it says “594 x 841 pt = 210 x 297 mm”
So I feel like every time creating a new score I have to manually adjust to the correct values of 210 mm x 297 mm for A4.
Is this a bug due to rounding in the conversion of measurement units?
It’s a consequence of the conversion, rather than a bug.
Dorico uses points internally. So it starts with 594 x 841 pt, and then converts it to mm.
Interestingly, the PDFs that I export from Dorico come out 1pt larger on each side:
595 x 842 is probably the closest that you can get to 210 x 297mm (209.9 x 297.04), and the internet will tell you that A4 in points is that size.
However, on MacOS, an A4 document is usually 595 x 841 pt. A4 PDFs from Adobe InDesign are also this size. Computers usually work to points, rather than millimetre (or inches).
A sheet of A0 paper is 841 x 1189 mm (1 square metre), and if you cut in half 4 times, you get 210.25 x 297.25 mm. Though in reality, the cut itself will have a width, reducing these values.
Either way, we’re talking about fractions of a millimetre. A piece of paper will vary in size by more than that, depending on the humidity!
You can of course edit the page size in the Page Size Editor.
Apple’s whole graphics system works in coordinates that map to points – I don’t know about Windows, but I’ll bet that whatever coordinate system it uses, it’s not based on millimetres.
I believe the ISO standard for A series is plus or minus 1mm anyway. I’ll have to look that up,. but for all practical purposes e.g. the paper you buy in office shops 209.55 is so close to 210 it does not matter, and within the standard specification.
We use A4 here in Australia and I have never found this to be a problem in practice.
