I know I'm quite late to the party, but I actually keep 100% of the source code open. The only parts I keep in a secret repo are tied to keys, hostnames and addresses, and certificates. But, just having a secret repo where sensitive information (including proprietary algorithms) was enough to satisfy the few investors I talked to.

Mind you, my next project is closed source, as I plan to open-source reusable pieces that will actually help people instead of a massive codebase that few have the means to run.