Too busy and medicated to read all the posts above but I saw something about water level. Ive owned the real piece, the 65t and actually the 60t, along with the bubbler version that the fc-187 is modeled after. They are all great and function slighlty better then the china versions, and obviously more aesthetically pleasing however the 186 and 187 are very well made and very thick.
That being said for water level..
The 187 should be fully covered, more restrictive stuff fill it right above the perc just covering, less restrictive can go about 1/4 to a 1/2 inch above the perc due to less drag from the attachment.
For the 186 I always fill it to the half way line of the second perc. This is HOW the american version was intended to be filled. When you pull it the water from the 90 degree joint pulls into the perc aswell. The bottom perc is designed to push water upwards, while the top perc filters it. The american version (60t mobius) and the 186, hit very SIMILAR. And I will admit I tried combusting out of my 186, and I would say it is just about as smooth as the 60t, which is one of the smoothest single perc flower tubes you can get in my opinion ( and i have ownd lotssss of american scientifc glass including SG, mobius, dirty rico, regredably toro, and a few others)
Edit: it is possible to drown a perc out, thats whats will happen if you cover both matrics. To each there own.