Well, I have expressed how I think your methodology is flawed. You take a number that is mostly caused by a certain number of conditions and correlate that with the entire JW population to come up with a number. If all JWs were exclusively having those conditions then yes your methodology would be correct but since they are not then it is flawed.
Again I think you are doing something noble for trying to come up with a number but there is a number of factors that you have to put into your evaluation to come up with an educated estimate.
Edit: Also as you state it doesn't include third world countries. That would have it's own complications even if all JWs took blood, such as the acquisition, storage and administration of the blood. Even a simple thing as administering a vaccine is complicated in third world countries let alone the storage and administration of blood which is much more complicated. Such as blood having a very specific best used by, has to be stored in very specific and complex situations, the testing of the blood for both pathogens and type for proper administration and the need for qualified professionals to actually administer it.