It's common to use a shorter stem with bullhorns than you would with other bars so I'm thinking 110mm is much too long. The only way to really get it right is through trial and error. Maybe a local shop has some used stems laying around they'd be willing to let you experiment with?

Next time you ride, try to imagine having your hands 10mm, 20mm, 30mm or 40mm closer. You should be able to get a feel for what seems right and go from there.