as I'm still fighting with layout/typesetting which should be based on
a baseline grid, I'm wondering how TeX calculates.

I've looked into TeX.web where I found the following definitions:
in := set_conversion(7227)(100)
pc := set_conversion(12)(1)
cm := set_conversion(7227)(254)
mm := set_conversion(7227)(2540)
bp := set_conversion(7227)(7200)
dd := set_conversion(1238)(1157)
cc := set_conversion(14856)(1157)

(Of course things get more complicated if---like an old European like
me---wants to change the base unit system to, say Didot points.)

On occasion I need to trace TeX's page building via \tracingpages=2.
Assume you have a \textheight of 39 \baselineskip + \topskip, and one
\baselineskip is 13bp. Then \textheight is 507bp+10pt. Using the above
definition for bp this should be 518.90125pt. But when tracing the
pages I always got a pagegoal of 518.90073pt resulting often in
underfull \vbox messages (I've hopefully eliminated all uncontrolled
stretching and shrinking of vertical material in my setup).

Maybe someone could explain this behavior to me?

Best regards,
Ulrich Dirr