What I do is sort of like homework. It takes some time at first, but then I know the answer.
First, I need a pool - in your neck of the woods this time of year you need a covered pool. In just a swimsuit, mask, snorkel and fins, I determine my body's buoyancy by seeing how much weight it takes to let me sink when I exhale about half way. Two pounds. I write that down. Next, I take my BC and see how much it takes to sink that (if anything), 1 pound, and write that down. Same for each piece of gear I have that floats, writing down the item and its weight requirement - I use regular scuba weights, and I have seven quarter-pound fishing weights as well, so I record each item to the nearest quarter pound.
For items that sink, I have a balance rigged that hangs under the diving board, where I can hang weights in a bag (under water) on one side and the item I'm measuring on the other and see how much negative buoyancy an item has, then I write that down on my list as a negative number. For tanks I measure them at 500 psi (my chosen number - some folks do 'em empty) with regulator attached (I don't figure I'll be carrying too many tanks without regulators on a dive). I end up with a list in my logbook that has the buoyancy of each item - so when I get ready to go diving, all I need to do is add up the values for me and all the gear I'm wearing/carrying, and I have my accurate fresh water weighting requirement. I then add 2.5 pounds per hundred pounds total weight (me and all the gear and the fresh water weights) to get my accurate salt water weighting requirement.
Looking at my list, for the question you asked, for me it would add up like this:
Me - 2
AL 80 with reg - 1
BC - 1
5 mil shortie - 6
Fresh water total - 10 pounds required
Total weight - 250X.025 = 6 pounds for salt water
Salt water total - 16 pounds required
Using this method takes all the guesswork out of weighting, and avoids unpleasant surprises at your safety stop - especially in Cozumel, where you'll be making them on the drift without a line to hold you down.
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Dry suit divers - it's a bit tougher as you have to measure the buoyancy of the dry suit with you in it, and with various combinations of underwear - time consuming, sweaty work. But once done, it's done and you never have to guess at weighting again.
Rick