Opinions abound regarding whether a BK false bottom is needed or not in the BK--but I won't get into that; I don't see the logic in using one in the BK, so I don't use one. JMO.
I use a sanitized SS Bullion China Cap strainer (or fine mesh nylon paint strainer bag) and large funnel with fine mesh screen to strain out any hop particles that make it out of the kettle. Never have had a clogged valve using this approach, but some of my brew buddies have cursed themselves for having a FB or Bazooka Screen on the inside of the kettle and having it get clogged during transfer. YMMV.
Measure from the inside of the kettle, not the outside of the kettle--the thickness of the triclad base on SS kettles may vary from brand to brand, and Aluminum kettles won't have one. So, to be consistent regardless of kettle type/material, measure from the inside of the kettle.
The weldless bulkhead portion of the spigot has a large washer and coupler that goes on after the O-ring on the inside of the kettle. Look at the image to see what I'm talking about--IMO, the washer determines your height.http://cgi.ebay.com/Weldless-Stainless-Steel-Valve-and-Spigot-Free-Shipping-/380122770975?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item5881152e1f
This was the one I used.
That will determine how high up you should go from the bottom of the inside of the kettle
. It is about an inch and a quarter, or so, IIRC.
At a minimum, it should be at least half the diameter (i.e., the radius) of the washer high up from the bottom of the inside of the kettle
starting from where it stops curving/sloping towards the bottom of the kettle. If you don't, then you'll have to trim/truncate one edge of the washer--that's unnecessary work. You can always make or buy a dip tube attachment later if you want to suck up every last drop of wort. Having it up just 1/4 inch higher higher than the minimum makes it easier to assemble and disassemble, IMO. If you do go with a FB, then build around that.
From the outside of the kettle, it looks more like 1.75 inches up from the bottom (as I have a SS kettle with a triclad base), IIRC.