White balance is certainly a challenging topic, and I'm still trying to figure out how to gain the knowledge and experience that'll suit me best. That said, I've settled on a fairly robust method that works quite well for me. Unfortunately, it's a little tougher than just doing a custom white balance, but it makes all of the difference when shooting flash.


Step 1: Set white balance to Kelvin (K), using a random guess (or just 5000) to start.


Step 2: Shoot a picture of the gray card (preferably ONLY the gray card) in the light that you're subject's in.


Step 3: Review the shot, and bring up the RGB histogram.


Step 4: If the Red channel is brighter (further right) than the Blue channel, choose a lower Kelvin temp. Choose a higher temp if Red is darker (further left) than the Blue channel.


Step 5: Return to step 2 and repeat 2-4 until Red is as close to Blue as you can get it.


Step 6: While still reviewing your most recent image, if the Green channel is brighter than the Red/Blue channels, you'll need to dial in some minus Green in the White Balance shift menu. If Green is darker than Red/Blue, you'll need to add some Green in the WB Shift menu.


Step 7: Repeat your cycle a few more times to get green balanced with red/blue.


Now you have a neutral white balance and you know what temperature you're shooting in. Since flashes fire at daylight (5000? 5600?) color temp and you've identified what color temp your ambient is, you'll now know exactly what gel to put on your flash to make it match (or warmer than) ambient light.