38DDD want to be a low C. How much weight should I lose before being considered for a reduction?

Hello and thank you for your question.
It is best to be at a healthy, stable weight prior to surgery. Your BMI should be no higher than 32. Schedule a consultation with a board certified plastic surgeon to discuss your options. Best wishes!

Your height will determine what your goal weight should be. Check your BMI on an online calculator to see what you should lose to get to a BMI of 30 or less. Consult with a board certified plastic surgeon to learn what options are available to you. Good luck.

The best way to determine what your breast size should be with respect to your anatomy and give true advice would be an in-person exam with a board certified plastic surgeon who performs hundreds of breast procedures annually. Not only does proportion depend upon the aesthetic balance of your breasts with your shoulders, waist, hips, and buttocks, but also upon your objectives, active lifestyle, soft tissue quality, muscular anatomy, and long-term concerns. There is also a limit to how small you can go due to tissue that needs to be retained for the nipple/areola to maintain a good blood and sensory nerve supply.
The surgery will also help to reshape and recontour your breasts. Very large breasts have the natural tendency to sag or hang low, so breast reduction surgery will lift them up and help them look perkier under tight clothing. The gram amount is dependent upon the density of the breast tissue and the volume removed. No direct correlation exists with cup sizes. You and your board certified PS can determine whether your goal "size" is realistic.
The most common breast reduction procedure typically requires a Wise pattern skin removal pattern to life the breast.  This is referred to as an anchor lift or inverted T lift. See breast reduction before and after photo galleries on my website to gain a sense of the scar pattern.