What is the equivalent CC for 300cc submuscular implant if you chose to go subglandular?

There is no standard "equivalent" volume difference when comparing breast implants placed in the sub muscular versus sub glandular position.  Make sure, as you move forward, that you understand the pros/cons associated with breast implant positioning- you will find a lot of information on the Real self website in this regard.My best advice: more communication.
It is definitely worth spending more time (prior to the date of surgery) with your plastic surgeon to discuss further.  As you know, it will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
One question that might be helpful for you to consider: would you be more disappointed if you are slightly too big or slightly too small after the breast augmentation procedure?
Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging) as well as careful measurements (dimensional planning) will be critical.Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, I have found that the use of words such as “natural” or "C or D cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this helps. Best wishes for an outcome that you will be very pleased with. 

Hello, and thank you for your question.  The difference might be small as far as size.  However the overall appearance can be very different when the placement is suglandular versus submuscular.  I would recommend a board certified plastic surgeon with vast experience to design a plan that is tailored specifically to you. Best wishes,Franziska Huettner

Submuscular placement affords placement of smooth implants, less long term rippling, greater camouflage, and probably a reduction in tissue thinning over time. Most patients with small breasts are better served by a submuscular augmentation to permit a more natural breast takeoff and better camouflage for the implant. The compression factor you reference is generally associated with under rather than over the muscle placement, not dual plane.  Very few breast augmentation patients have enough breast tissue to permit a subglandular augmentation. Slightly decreased projection may be noted following submuscular placement, but no actual loss of volume.