> I am presuming that tk truncates fractional parts as int() does; if not, use round().

Tk uses both truncated and rounded value.

$ rlwrap wish
% canvas .c
.c
% .c configure -width 10.1
% .c configure -width
-width width Width 10c 10
% .c configure -width 10.9
% .c configure -width
-width width Width 10c 11

As you can see 10.9 gives both 10c and 11.