I have this METAPOST macro:

vardef makeTeXLabel( expr w, h, name) =
  if debugProgram or debugLabels: show "NAME makeTeXLabel:", name, tostring 0.8w, tostring 0.8h; fi
  save p; picture p;
  p := textext( "\startframedtext[middle]" &
    "[align=middle,frame=on" &
    ",width=" & tostring 0.8w & "bp" &
    ",height=" & tostring 0.8h & "bp]" &
    name & "\stopframedtext");
  save b; path b; b := boundingbox p;
  save dim; pair dim;
  dim := (urcorner b) - (llcorner b);
  save width, height; numeric width, height;
  width := xpart dim;
  height = ypart dim;
  if debugLabels: show "PLACE:", w, h, width, height; fi
  if (width > 0.8w) or (height > 0.8h):
    if (width > height):
      if debugLabels: show "XSCALING:", (0.8w/width,0.8w/width); fi
      p:= p xyscaled (0.8w/width,0.8w/width);
    else:
      if debugLabels: show "YSCALING:", (0.8h/height,0.8h/height); fi
      p:= p xyscaled (0.8h/height,0.8h/height);
    fi
  fi
  p
enddef;

It gets called with w=133 and h=53 (I am working in METAPOST native units, so bp)

So, the textext() command gets something like 106bp x 42bp, which seems to be the case:

metapost log    > >> "NAME makeTeXLabel:"
metapost log    > >> "My 'Application' Function"
metapost log    > >> "106.40000000000001"
cld             > tex > w : - : \MPLIBsetNtextX{1}{\startframedtext [middle][align=middle,frame=on,width=106.40000000000001bp,height=42.400000000000006bp]My 'Application' Function\stopframedtext }
cld             > tex > F : - : \startblankhandling
cld             > tex > w : - : \setblankcategory{6}
cld             > tex > F : - : \flushblankhandling
cld             > tex > F : - : \stopblankhandling
metapost log    > >> "42.400000000000006"
metapost log    > >> "PLACE:"
metapost log    > >> 133
metapost log    > >> 53
metapost log    > >> 511.08659908200002
metapost log    > >> 42.399990514000002

But the bounding box is 511bp wide (something like 7in or 18cm, probably textwidth) but I’m asking for a box that is 106bp wide.

What happens here? And how do I get it right? 

G