Agree with SG, also you don't always need to be deep. I would rather get a good swim of the fly than a poor swim on the bottom
In heavy deep water, a heavier fly gets down yet retains action because of the turbulence. Example: a fast riffle flowing over a ledge into a deep pocket. You have to drop into the zone fast.
In soft deep water, a heavier fly sinks like a stone but doesn't have any action because there isn't enough turbulence to swim it. Example: further down the run in the previous example where the depth is still there but it is slowing and approaching the tail out. You have more time to get the fly down and a slower swing.
Another rule of thumb I use is that I don't want a sink tip that sinks faster there my fly. So for that deep soft water I would probably run a longer medium sink tip like T8 with a medium weighted fly.
Yet another rule of thumb is pay attention to how long your fly fishes and use that to decide how fast of a tip, fly, leader length et cetera. There is always a sweep spot for each little piece of water where you can throw it out there and just let it fish. The hard part is keeping a fly fishing well all day without messing around with knots, leaders, and tips. This is why I feel experience with the particular water you are fishing and what is needed to fish it effectively is more valuable than any advice out there.