OK, I did a little more looking around and hopefully answered my (admittedly very electronics 101) question. Perhaps some sage and kind person could check my math and assumptions. Attached is a schematic of the V475-2A from Kubarth for reference.

Assumption #1: From what I understand, the value of Rk (called Re on the official Neumann schems) is 5K11 for all Nuemann summing cards, regardless of number of inputs. I will be starting with a 8x2 summing box - perhaps to expand to 16 some day.

Assumption#2: If I use -3dB pan law for center (3dB = 1.4), 5,110x1.4 = 7,154. So, I would need to add 2K resistor in series for each leg for mono channel assignment to get to -3dB +/-.

Assumption#3: Adding a resistor between high and low to minimize crosstalk is not necessary for Neumann active summing card.