How do you effectively debug overfull hbox warnings?

I still haven’t found out how to effectively solve Overfull hbox (0.80913pt too wide) has occurred while output is active warnings.

For instance, this piece of latex generates such a warning and I am stumped why:

newcommandHeader{%
noindentmakebox[textwidth][l]{
parbox{0.5textwidth}{contactinfo}
parbox{0.5textwidth}{%
hfillincludegraphics[height=25pt]{logo}}}parvskip1ex%
parColTopRulepar}

contactinfo is defined as follows:

newcommandcontactinfo{fontsize{8pt}{8}selectfontheaderfnt Streetname 12324  PostalCode City  +31655555555555}

Error:

*geometry detected driver: pdftex*
<logo.eps>
Overfull hbox (0.80913pt too wide) has occurred while output is active
 [] 
[1] <logo.eps>
Overfull hbox (0.80913pt too wide) has occurred while output is active
 [] 
[2] <logo.eps>
Overfull hbox (0.80913pt too wide) has occurred while output is active
 [] 
[3] <logo.eps>
Overfull hbox (0.80913pt too wide) has occurred while output is active
 [] 

Expected output is:

Streetname 12324                                       here comes
PostalCode City                                          the logo
+31655555555555                                     right aligned
-----------------------------------------------------------------

I mean, both boxes have a length of 0.5textwidth, the text is obviously smaller than this, as is the logo. How come it still generates such a warning? And how can I found out why?

showboxdepth=maxdimen
showboxbreadth=maxdimen

showboxdepth=5
showboxbreadth=5