Need advice using Expodisc with flash

I've used the expodisc in the past and it has the ability to yield very accurate results when using available light.

Does anyone have any tips on how they've successfully used their expodiscs to measure white balance when using flash in varying situations (i.e. fill flash outdoors; indoors when bouncing the flash isn't feasible and there's no mirror to shoot at). Thanks in advance for your tips on using flash with the expodisc especially in the scenarios mentioned.