Quote Originally Posted by locutus View Post
Do you have any rule of thumb for in camera preflash?

My guess would be to start with some rule like 'preflash with diffuser for 1/3rd of the metered exposure' obviously one would have to bench that against the actual light transmittance of the filter etc.
Tim Layton has a discussion on youtube on preflashing with tracing paper like what I do (at about 2:20):

https://www.youtube.com/watch?v=N1nhN_057FE