On the KB calculator, it asks for intake closing point at 0.050" tappet lift plus 15 degrees as you stated. What they're suggesting is that there will be about 15 degrees of crankshaft rotation on the closing ramp from 0.050" tappet lift to the advertised duration closing point which is calculated at 0.006" tappet lift.

In other words, if you had a cam with 220 degrees of duration at 0.050" and 250 degrees advertised duration at 0.006", the difference would be 30 degrees, 15 degrees on the opening ramp and 15 degrees on the closing ramp. So to get to the point where compression can actually start (intake valve on the seat), they use an arbitrary figure of 15 degrees to be added to the 0.050" figure.

Personally, I thing the KB calculator is conservative, so I still use it as they suggest, but instead of looking for a DCR of 8.00 to 8.25, I have adjusted my personal choice to between 8.25 and 8.50. That makes up for the conservancy in my opinion.

Shawnlee recently built his 305 to a DCR of 8.40, so I think he's thinking along the same lines as I am.