hello,
i dont use PWs.... but i do use phottix odins.... i found out that even when the speedlight or the ODIN transmitter/controller unit is set to manual you cant use a light meter to meter the light output.... tried this on my sekonic 478 and reading on light output were too low such that when you input those in your camera... images will be overexposed...... the workaround i found was to to trigger the flash via sync cord instead of via hotshoe on the receiver units.....
I'm not really clear on what the OP's question really is though..... I know sekonic has models that can trigger PWs..... so maybe OP was saying something that he can't accurately measure the light even when in manual when using ETTL PWs..... if that's the case Patrick maybe you could try the "my solution" as i posted above.... to clarify though i use phottix odin though i believe they work in same ways as PWs tt5/tt1.....
good luck