Yeah, you can do that. What I found from a practical perspective is I usually play with X -sync speed to ensure flash to ambient ratio is approx 4:1. At this ratio ambient becomes a non issue really. And stop down to ensure that at The x-sync Selected and f number ambient gives very little exposure. ISO varies from 200 to 400 usually.I don’t understand why that would give you natural skin tones, it flies in the face of what ETTL II is doing and depending on actual EV would result in a 2 stop over exposed subject.
of course nobody knows the true details of ETTL II and there is a movement of exposure bias depending on the EV. The only reliable way to shoot in mixed lighting with flash is to go manual everything. In dynamic situations, like receptions etc, ETTL II works remarkably well, but for set up portrait shoots ETTL II is unpredictable because of its proprietary nature.
then I would set Flash exposure compensation to a little over say 1/4 to 1/2 stops.
with high ceilings flashbender XL Pro plus on camera flash is. gelling is rarely an option as light quality changes as you move around. A lot. Therefore it is either mixing available ambient with focus in skin tones in post or removing ambient all together. Nothing is perfect though. Run and gun setups is always a compromise however this little strategy affords me pretty consistent results.