Question on stems....

I currently have a single-speed "rain bike" with bullhorns. Its stem is 110mm and 7 degrees. I feel the reach is a bit too much since I can feel the tendons ache in my shoulder and elbow. I don't have this issue with my road bike (same frame size, just with drops)

I'd rather keep the bullhorns on the SS and wanted to know if I went with a shorter stem, would the degrees also change.

If so, how do I decide on what stem to get 100mm or 90mm or....is it better to go to a LBS and have them figure it out for me?

Thanks Scrod!