Let me make an alternate proposal. Suppose that we want 600 congresscritters and the population of voters is 300M. Dividing A into B, we get a simple ratio of 500K to one. Let's allow them to sign up directly. No reason for it to be limited geographically. If you like Representative Archie, sign up with him. When Archie gets 500,000 registrations, he gets a seat. Even better, we'll let more people sign up -- if Archie is so popular that a million people sign up for him, then he gets two votes. We'll stop there to prevent too much power concentration -- more people can sign up for him if they like, but he doesn't get extra votes.
Once per period, all of Archie's constituents are surveyed to determine if they still like what he's doing. They can change registration at any time, but this is the most convenient time to do it.
If Archie drops below 1M at any time, he goes back to one vote. If he drops below 500K at a survey point, he's no longer a Representative.