We are now leaving the realm of your current client roster (and the place where you can estimate real numbers); and entering new territory which potential employers scrutinize heavily and with a decent amount of skepticism. Hence, you need to be as specific as possible, and inspire trust and confidence.
I regularly see BPs where partners endlessly discuss all the great clients that they will add to their roster once they have the right platform. Often these partners even give the names of the entities and the titles (but not the identities of course) of their contacts at these entities. Make no mistake: it is wholly insufficient to discuss all the great things you will do someday in your BP. It is imperative that you have a section in your BP where you discuss the ways in which a new platform will allow you to increase your client roster and portables, but you must already have begun to take steps to make this happen so that you can add real information to your BP. If you have contacts at various entities that you cannot presently service due to the limitations of your present platform, then meet with those contacts at those entities and determine how much business you could reasonably expect from them if you moved to a different platform.
I once worked with a construction partner whose portables were not particularly compelling, but who had a lot of potential portables if he could find a platform that could service more sophisticated work. His immediate goal was to secure a very big international real estate developer as his client; and he knew that in order to do so, he needed to move to an AmLaw 100 firm. This partner had many meetings with this international real estate developer, and when he drafted his BP, he was able to include very specific information about the amount of business that this client would give him if he was able to secure the proper platform. His search was very successful and this partner - based upon the estimated portables of a client with whom he had never worked - was able to make the transition from a very small real estate boutique to a well-known AmLaw 100 firm.
The moral of the story is that you must not wait to meet with these potential clients. If you meet with them now, and get some indication that they will work with you if you make a move to a platform that can handle their business, then you can include these clients in your BP; and thereby increase your estimated portables. Plus, you differentiate yourself immediately from the dreamers who ramble endlessly about how things will be someday by demonstrating that you have initiative and have already taken action.