I have a 90g fish tank...my method:
I use a 32 gallon plastic garbage can marked (FISH ONLY)
I use 5g and 3g jugs of RO water (measure is easier when adding my salt) I just count how many jugs, my SG is usually right on target.
A heater and power head to mix things up for 24 hours
Use the power head as a pump and plastic tubing to pump out and in the water, when making the change...
Hope this helps you...Happy reefing!