The way to guarantee equal lift is to have independent lines for bow and stern. This isn't a great photo, but shows another version of grayhawk's hoist:
There are two lines, each anchored at the top, down to a pulley at the boat cradle, up to a second pulley (hung from the same anchor) and off to the haul point. The lines are knotted together where you haul them to guarantee equal lift. Actually they're knotted to a heavy duty steel ring that hooks to a cleat when the boat is at its proper height. This setup gives a 2:1 mechanical advantage.
Forget all the stuff above about friction and the number of pulleys; it may be true but is not reliable enough to base a design upon (sorry GBG).
EZ-Dock modular docks
Deck Rigging Gear
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