How much smaller will I be if I'm a 34k and insurance requires 800 grams to be removed on each side during BR?

Congratulations on your decision to proceed with breast reduction surgery. It is one of the most patient pleasing operations we perform.  There is no direct correlation between the amount of tissue removed and the ultimate cup size that a patient will wear after breast reduction surgery.

Before undergoing the breast reduction procedure it is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C or D cup”  means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.

Best wishes.

usually end up being D+ in the end... as it takes considerable effort to get down to a C cup and its unheard of that a B cup results from a standard reduction.  Ask your surgeon for reassurances that you will maintain a certain size and you could also ask your surgeon to limit the resection to the insurance minimums to preserve as much of your native tissue as possible.

There is just no direct correlation between grams of breast tissue and cup size. Your plastic surgeon can make an estimate of the amount of breast tissue to be removed, but it is only an estimate. The actual volume of breast tissue removed is determined at the time of surgery, based upon the needs at that time. Breast reduction is taught to plastic surgeons as an  'aesthetic' procedure, not a weight-reduction operation. Your plastic surgeon is your best resource in answering questions about the procedure and your specific goals regarding shape and cup size.

Here's what you need to do.  When you go in for a consultation, have the plastic surgeon show you an 800 cc implant (that's a big implant!) and hold it up next to your breast.  Do mental subtraction to get some idea of what sort of volume you will be looking at after surgery.  One bit of unsolicited advice - do not let insurance coverage get in the way of a nice and balanced result.  I have seen several women over the years who were way over reduced to meet some arbitrary insurance company requirement.  Big mistake.  They would have been much better off saving up and paying for a smaller reduction out of pocket instead of having further surgery to restore a reasonable and balanced chest.

Weight and cup size do not exactly correlate. If you have 1600gm breasts, of course taking off 800gm would be a pretty reasonable reduction. If you have 1000gm breasts, taking off 800gm would ruin the breasts. We have no way of knowing how big you are (your clothing size is not helpful). Get an exam by an expert for answers.