These discussions keep happening because PADI's "float at eye level" training doesn't come with the rationale behind it. That methodology is for the population that PADI is most oriented to: the occasional diver in a 3-5mm vacation wetsuit. If you're in cold water, everything changes due to your neoprene.
Where you float with an empty bc, and how big a breath you retain, and full or empty tank, ALL relate to only two requirements: being able to float at the surface with a failed bcd, and being neutral at the 10-15ft stop with no air in your bcd. Everything else devolves from this.
Why do the requirements of the buoyancy tests that folks are taught vary? Because the neoprene varies.
Example 1): 3-5 mm wetsuit. At 15 feet, that wetsuit will lose ~5lb of buoyancy due to suit compression. If you finish your dive with min gas, and need to hold your safety stop with a failed bcd (or no air in it), you have to weight for that suit compression. At the surface, where you do your buoyancy check, you are 5# light due to suit expansion. If you weight yourself to float at eye level with a 500 psi tank, you are supporting approx 4 lb of skull weight above the surface, and will be approximately neutral at the safety stop. Especially since most divers have more than an "end expiratory" lung volume during the test due to a little stress.
Where it gets complicated is that OW courses NEVER give you a 500 psi tank to test this for 3mm. So you're 6# heavier, and won't even float awash without full lungs. Stupid to not discuss the theory. What I teach is to weight just as above (but with a full tank), then add weight for the gas you'll breathe. Typically 5-6 lb. Problem with this on vacation with a new setup is that you have to be prepared to jump off the boat and do your quick weight check with a full tank, and then quickly add 6 more pounds, all the while your buddies have descended and left you behind. No wonder it never works out right.
Example 2): 7mm wetsuit or maybe a Farmer John. At 15 feet, that wetsuit may lose 12# of buoyancy due to compression. To do a surface buoyancy check that will make you neutral at the safety stop you have to account for 12# of buoyancy that you won't have at 15' at the end of your dive with a failed bcd. If you start with a full tank, there's 6-8 lb that you won't have at the end of the dive. If you float at eye level, there's another 4# less than neutral. If you have slightly more air in your lungs, that gives you the extra buoyancy to make the test valid, and account for suit compression.
At the end of a dive, you are weighted for the buoyancy of your neoprene AT 10-15 FT, so you don't need bcd air to hold your stop, and you don't need extra weight for the near-empty tank. It all balances out.
But when the test is performed at the surface and PADI doesn't teach the theory of suit expansion, we get six different methods, and they're all correct.
Different neoprene, different tests.
Now for the end of the dive with the failed bcd. With thick neoprene, it's easy. You're 10-12# light at the surface and can float easily. For the vacation diver in 3mm who just passed his/her OW, you're at eye level being 4-5# light. A big breath and you're easily floating on your back. But in a panic, that may not seem good enough. Hence, ditchable weight.
Coincidentally, this relates to the OP's problem. If you're correctly weighted in heavy neoprene, even with a full tank you're 4lb light at the surface at the beginning of the dive, unless your lungs are empty and your head is just awash (which is not the way divers like to rest). So unless you exhale more than four lb of buoyancy and hold that until suit compression takes over, you won't sink. Starting vertical, to compress those legs and squeeze bubbles up is a good start. Exhale hard, don't panic, wait 15 sec for negative buoyancy to take over, and slowly rotate to horizontal as you continue your descent. Don't take a huge breath close to the surface. Problem solved.
With a dry suit, it gets a little more complicated, but not much, since you always keep the same (relative) volume of air in the suit if you use it for your buoyancy control. The problem with drysuits is that at the start you may need to carry 6-12# more buoyancy for your full tank(s). If you don't, you'll have some squeeze at the end trying to stay neutral. Alternatively, start your dive with 3-6 liters of air in the bcd and bleed that off as you use gas. It gets complicated as teaching varies re: which device is the buoyancy adjuster under way.
