The issue of how much weight I need comes up every time I make significant changes to undergarments or use unfamiliar tanks.
Here is my approach:
The goal of correct pre-dive weighting is to be just a tad negative when the tanks are nearly empty after venting the suit in our normal ascent position.
Unfortunately, tanks are usually full before we start the dive and we have to predict what will happen when they are (nearly) empty.
We know how much weight we loose when we breathe down a tank. I use the figure of 8# per 100 cuft. If I dive a HP100 tank I better be 8# heavy at the beginning of the dive.
How can I find out how much negative I am at the beginning of the dive? I could drop weights until I am neutral and then put the desired amount (in this case 8#) back on for the dive or I could add 8# of buoyancy and see whether I am still slightly negative. I prefer the latter by orally inflating my wing, knowing that my full lung volume creates about 8# of buoyancy.
Enter shallow water fully kitted up with regulator in mouth and mask on.
Float on the surface, face down, with the drysuit somewhat inflated and the dump valve closed.
Wiggle and stretch and verify that I can reach the manifold valves.
Open dump valve and dump from wing (or BC) to submerge.
I should be heavy enough to sink to the bottom as the tanks are still full.
On the bottom, I first dump every little bit of gas out of the wing, then slightly inflate my suit and 'roll' as much gas out of the suit as I could during a horizontal ascent.
I take a deeeeep breath and then blow my entire lung volume into the oral inflator of the wing. Once I resume normal breathing from the regulator I am now 8# more buoyant than before.
If I am barely neutral at this point, my weighing is OK for 100 cuft of air/nitrox. (200 cuft will need two full breath in the wing, of course)
If I am still "stuck" to the bottom, I know that I am overweight. If I cork while orally inflating, I need more weight to be neutral at the end of the dive.
The whole procedure takes less than 3 minutes and works in fresh and salt water.
With some practice this can even be done without a bottom.
Drain the wing on the surface while being vertical in the water and use suit inflation to keep you from sinking.
Blow the desired amount of gas into the wing.
Do the surface wiggle in the suit, dump the gas from the suit and see whether you slowly sink.
I typically exhale fully and kick down a few feet to see whether I can stay there in horizontal trim with normal breathing.
How do I determine my lung volume? The easiest would be to ask a physician during your next checkup.
If your doctor does not have a spirometer or you are the DIY type here is one possibility: Fill a gallon jug with water and hold it in a water filled sink with the open end facing down. Now, blow air into the jug through a piece of vinyl tubing, garden hose, etc. How far you can fill the jug with air will give you a pretty good idea of your lung volume. Larger people may need a larger vessel of known volume or multiple jugs. Don't worry about the third decimal, we just need a rough idea.
Edit: Agree with James. Erring a couple (!) pounds on the heavy side is not a bad idea. However, I found that the method I described above errs towards slightly heavy as the whole kit gets a little less buoyant in the first minutes of the subsequent dive due to all the little bubbles coming out of every nook and cranny.
