I recently started squatting as part of the Starting Strength program by Rippetoe.
So when I go real deep, as in ass to calves I round my back.
I’m 6’4’’ and I have fairly good hamstring mobility (forehead to knee) and i would like to be able to squat deep without rounding my lower back.
Are there any exrecises i can try to fix the depth?
Squat wider, being taller usually means you might have to squat wider unless your pretty flexible in the hips and ankles.
watch all of these also, they are helpful for developing a perfect squat.
fix your glute mobility with foam rolling and tennis ball(then harder balls) tissue (myofascial release) work against the wall, and later on the floor
Also do the bottom of the feet as well, the fascia connects all the muscles together.
Your probably tight everywhere 
see my posts here for more info
http://www.T-Nation.com/tmagnum/readTopic.do?id=2232908
then work on glute activation and lower ab strength
rounding of lower back has nothing to do hamstring mobility. Pull your knee to your chest - what limits your ability to get your knees high without your lower back rounding? It’s the glutes and lower ab strength