$ ack -ri HeightMMOfScreen tk8.6.11 -B1
tk8.6.11\generic\tkCmds.c
843-    WidthMMOfScreen(screenPtr) = width;
844:    HeightMMOfScreen(screenPtr) = height;
--
1485-   Tcl_SetObjResult(interp,
1486:           Tcl_NewIntObj(HeightMMOfScreen(Tk_Screen(tkwin))));

tk8.6.11\unix\tkUnixEvent.c
201-    }
202:    if (HeightMMOfScreen(DefaultScreenOfDisplay(display)) <= 0) {
--
205-    mm = HeightOfScreen(DefaultScreenOfDisplay(display)) * (25.4 / 75.0);
206:    HeightMMOfScreen(DefaultScreenOfDisplay(display)) = mm;

tk8.6.11\xlib\X11\Xlib.h
115-#define WidthMMOfScreen(s)  ((s)->mwidth)
116:#define HeightMMOfScreen(s) ((s)->mheight)