For those who are interested, I got a meter reading of 50/s with f11 and exposed 3 sheets of film f8/11/16. Turns out the Graflex does like being overexposed. F8 could even have done with a little bit more.

So, for reference, +1.5 stops works well. This confirms the advice I had received from people before regarding large format and generally needing to overexpose.