Playing in tune is simple. Not necessarily easy, but simple.
Often, I see the “I sound out of tune, thus my slides must move” approach. While that is a part of the equation, the most important things you must focus on are not whether your slide is at 3/8″ or 5/16″. Your best tone and air will give you the best indication of where the pitch is. If you play with poor sound, you could be pinching the note sharp, and blowing it flat, and there is where the slide pulling is really ineffective.
Rely on your ears, they are usually more reliable than what is in between them. As far as inconsistency goes, make sure to simplify: stop, listen and digest the pitch, then play the note relaxed and with your best sound.
Tone is as much a vital part of pitch as slide length is. Once you play with your best sound, then you can play with good inTONEation.
As Anthony Maiello says “Intonation is like body odor: everyone has it, most people do something about it.”