Converting byte position to character position

The context to this question involves GTK2.Pango.Layout's
xy_to_index() method, which returns the byte index into a (presumably)
UTF-8 encoded version of the string. But it's really a string
manipulation question, independent of GTK.

What's the easiest/simplest/cleanest/best way to convert a byte
position into a character position? My current solution looks like
this:

string txt = "Sermon \u201cLife Changed by an Encounter with Jesus\u201d (IBO)";
int pos = 10;
int charpos = sizeof(utf8_to_string(string_to_utf8(txt)[..pos-1]));
// and charpos ends up 8, which is correct.

And it works, but ow! what a mess. I could alternatively go through
the string manually, counting up by byte range (<127 = add one, <0x800
= add two, etc), but I suspect that'll work out even messier. Is there
a cleaner way to do this?

ChrisA