i still think it is worth the time and materials to do the sample sewing to see what the " finished" size will br.
some variables that can affect the finished size besides the seam width
the size of the cut piece - how is the ruler lined up on the fabric? using die- cuts, strips cut from a june tailor or stripology ruler?
i use a ruler - and when i measure a strip, it is wider that the specifed width by the distance the rutary cutter is from the edge of the ruler.
as others have mentioned, the thickness of the thread and the thickness of the fabric also affect how much is " lost" when the unit is opened up.
also, how is the unit pressed? some people seem to leave a tiny pleat at the seam line instead of getting itbreaaly flat at the seam line.
i think some directions say "scant" to imply that the seam allowances do matter with that pattern.