How much suction to apply to skimmer?

This is kind of general question that I'm not sure has any quantitative answer but here it goes anyway -

My skimmer and main drains are tied together in series with your standard anti-suction float diverter in the skimmer. SO during normal operation the pump line attached to the skimmer is pulling on both the MDs and the skimmer. As well, I have a suction-side vacuum cleaner (Pentair KreepyKrauli) with it's own dedicated vacuum port. Those two lines (skimmer and vacuum) get attached to the pump through a three-way Pentair valve that is fully adjustable from 100% pull on skimmer to 100% pull on vacuum with any ratio in between.

Normally I leave KK in all day skittering about the bottom and I adjust the valve so that it's mostly (~90%) on the vacuum port. I still see water swirling in the skimmer and the Weir door stays open from the water draw. My surface skimming isn't as good when KK is running.

Sorry for the long-winded setup, but my question is this - how do I know I have "enough" suction on the skimmer? Is it just as simple as, "the weir door is open and the surface is clean enough"? I run my VS pump on low speed (2500RPM @ <1KW power) most of the time which is probably why the three-way has to be set so far towards the vacuum port. Not to mention the pipe run to the skimmer is probably a lot shorter than to the vacuum port so there's less head loss.

Is there something simple to look for to be able to say the skimmer is getting enough suction? Should I swim down to the MDs with a little red food dye to make sure they are actually sucking in water?

Any advice would be most appreciated...