My max deadlift (for now) is 435lbs, my above knee max rack lift is 605lbs, my just below knee max rack lift is 450lbs....does this indicate that my 'take off' is the weak part of the lift? and if so, what can I do to improve the bottom part of my dead lift?
Most people are much stronger above the knee than below. However, if you feel that the disparity is too great, there are a few potential problems you may want to check:
If your hips are too high, you are pulling from the most mechanically inefficient position. Additionally, it is putting a lot of strain on your back that can potentially be dangerous to your lumbar region.
The best thing to do is get some video of yourself to see what you look like. Take it from a couple different angles: 3/4 view on the front, and a side view. The 3/4 view is from the front but at an angle that also lets you see your leg and back position. A good example, as well as some professional deadlift critiquing would be in this video from Brandon Lilly: http://youtu.be/vdINRhBUvsM
You want to breathe into your spine. In other words when you take a deep breath, that breath should fill and brace your spinal position before you attempt a pull. Hold that breath until you are fully erect. Just breathing into your stomach or chest isn't going to help the spinal position.
If you don't have mobility or body position issues, there are some exercises that can really help:
Both of the above are assistance exercises I've used and have produced some good results. They shouldn't replace actually training the deadlift off the floor. Make sure you pull the bar toward you throughout the movement, activating your lats.