rounding error in simple assignment

Question

I have a simple assignment in LuaTex:

\dimen1 = 210mm

Now I print the value

\the\dimen1

The result is: 597.50787pt

Calculating back with 1pt = 0.351mm is: 209.725...mm. This is an error of more than a quarter mm. Seems not much, but saying \pagewidth = 210mm means, my page is a quarter mm too small.

When I do \dimen2 = 598.2905983pt (the correct value for 210mm), \the\dimen2 shows the nearly correct 598.2906pt.

Why is the error so unexpected large when assigning millimeters? Am I using the wrong factor between pt and mm? Or is 0.351 rounded itself?

Thanks for all answers Peter

Answer 1

Oh, I found the answer myself. 0,351 is a rounded value. Donald Knuth set his TeX-pt to exact 1⁄72.27inch which is near to 0.35145980mm. It's so easy :-)

Found the answer here: https://en.wikipedia.org/wiki/Point_(typography)

Thanks for reading Peter