BR: Getting 250g off of 34D cup. What size do you approximate I will be?

Getting a 250g off of a 34D cup size may result in a 34B plus cup size. This is a rough estimate that 100-150 grams is a 1 cup size depending upon your chest measurements. This means a female whole chest measurements are 24 or 28, 100 grams will be 1 cup size. But those whose circumference is more, 150 grams can be 1 cup size.
Once you discuss all these things, you can also request for a Crisalix 3D Simulation, so you can see what your results will look like before the surgery. 

Thank you for the question and congratulations on your decision to proceed with breast reduction surgery; this operation tends to be 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. There is simply too much variability among bra cup manufactures to make it a useful metric when it comes to preoperative communication and/or post operative predictions.
 Before undergoing the breast reduction procedure it will be very important to communicate your size goals with your surgeon (who you should choose very carefully). Most patients wish to achieve enough of a reduction to help with their symptoms while remaining proportionate with the remainder of their torso. 
With the goal of improving communication with my patients I find the use of photographs of “goal” pictures (and breasts that are too big or too small) very helpful. For example, I have found that the use of words such as “natural” or “B or C 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. 
Once you have communicated your goals carefully, your plastic surgeon will be able to provide you with an estimation of how much tissue will need to be removed, in order to achieve your desired outcome.
Best wishes with your breast reduction procedure; hopefully you will be very pleased with the outcome of the procedure performed.

While there is not a direct correlation between grams removed and cup size, I would say you can count on going down at least one and maybe two cup sizes.  I don't always pre determine the exact amount of tissue I am going to remove before surgery as some tissue is denser than others.  I am sure you will be very happy with your breast reduction!