Developing your ear is the best thing you'll ever do for yourself as a DB player. And you've gotta start from day one.
Put on your tape, your markings, whatever. Then practice intonating without using the markers. Just don't look. It's OK to sound like **** in the practice room. It's impossible to get good without sounding like **** so do us all a favour and do it in the practice room.
Pick a note. Play a half step up from that (slide, use another finger, whatever.) Slide up a whole tone. A minor third. A major third. Work your way up to octaves and more. Learn about the vomit exercise.
Do this work every time you practice.
Seriously, it won't be long before you can do it too. The faster you do away with the tuners and the tape, the better.
For a real rush, once you've done that pick up your fretless EBG again. Once you get used to moving from your DB sea legs to your EBG land legs again, the thing is now more like a child's toy to you than it ever was before. You'll have your way with it and it will be very cool.