No rules for names I guess, but you most likely mean Dieter. "ie" is called a "long i" in German and is pronounced as the name suggests, that is, like the "ee" in seen. Just out of curiosity: I see this very mistake happen frequently to native English speakers when using "long i"-words. Why is that? Does an "e" after an "i" look weird to you because it's a uncommon construct in English?

Probably the opposite -- "ie" is much more common in English than "ei", and so "ei" looks more German to us than "ie" does, thanks to words such as Einstein, Geiger, Klein, mein, Steinberg, and Zeiss. Of course none of those are pronounced anything like Dieter. It's much harder to think of an adopted German word or name with "ie" in it -- Siegfried is about the only one.

It also doesn't help that "dieter" is an English word with three syllables -- die-ate-uh, one who diets (i.e. with food) -- so it looks wrong to write it for a German word with two syllables.

Peter