For starters, that skimmer is marginally oversized for your tank. You could be dealing with an issue where the skimmer neck diameter is too large for your load and the lack of sufficient DOC's (for that skimmer) can make it impossible for the skimmer to maintain a consistent foam head. What ends up happening is that the skimmer will skim great until it lowers the DOC's at which point, the foam head will tend to lower until the DOC's build back up. I have run into this issue a number of times before with other people and even had this issue on my tank with an Alpha 300 before I upgraded to the smaller Supermarin 250 which ended up being much more consistent. FWIW, the BK skimmer size ratings are typically based on display size. Not overall water volume size. This is because the display is where the load is concentrated and your sump/refugium is actually part of the nutrient export.
That said, this skimmer should be in a sump depth range of a minimum 6" and a max of 7.8" with about 6.5" sump depth being optimal. As far as the number of turns out on the volute, 1.5" is a good starting point but you may need to go more or even less depending on how the foam looks. Here is a link to the manual. Take a look at the setup steps on page 3& 4. Particularly the internal water level on page 4. Make sure you sump depth is in the range mentioned above. The higher the water level in your sump, the wetter the skimmer will skim meaning larger bubbles higher up in the skimmers body. The idea is to get the sump level such that the bubbles inside the skimmer start at the optimal level (page 4 of the manual) with the wedge pipe wide open. From there you fine tune. Start by adjusting the volute to get a nice solid foam head. Once you have a good foam where the bubbles are not bursting at the surface, then fine tune using the wedge pipe. Keep in mind that if this skimmer is new, it will take up to 2 weeks for the skimmer to fully break in and settle down. Also keep in mind that when you make an adjustment, it will take an hour or two before the results of that adjustment are fully visible. As such, make one adjustment at a time and wait a couple hours before making another adjustment.
Also keep in mind that the adjust volute works as follows. Opening the volute allows more air but tends to create larger bubbles. This also allows less water into the pump as it's displaced by air. Closing the volute, lessens the amount of air but also decreases the bubble size which can be good. This also increases the amount of water going into the skimmer which tends to raise the water level. Finding the sweet spot just takes time and experimenting but your 1.5 turns out is a good starting point. Water level in the sump however is very critical.
Here is the manual.
http://royalexclusiv.net/images/content/bk_dc/eng/bk_dc_operating_maintenance_eng.pdf
