Lily units of length

[John Sankey]

I'm getting an inconsistency between points and millimeters with
Lily. According to international definitions, a point is 1/72 inch =
0.3528 mm. But, when I change the units of arithmetic_multiplier from
\pt to \mm, I need to use about 0.4 mm/pt to get the same results!

I *hope* that Lily uses mm internally in accordance with SI!
If it does, is a point defined correctly? (And, where is it defined?
I can't find it in the /usr/share/lilypond/ly/ files...)

John