The Nixie display board is a 10x4 matrix of IN-12B (ИН-12Б) Nixie tubes. Each tube has a 6x2 male header, where one pin is for +170VDC and 11 data pins are for the digits plus the dot. You keep all the data pins above 120VDC to keep the digits off, and pull one down to ground to turn it on.

The tubes are on one side, and the headers stick out the other side where they can be connected to drivers.

Since the calculator will have 20x4 tubes, we need two of these.

Here's the schematic for one of the 40 tubes:

I wired things up cleverly so that power was delivered in one corner, and then starting from the opposite corner and working your way back and forth, you have the pins for digits 0, 1, 2, ..., 9 and then the dot. The numbering of the pins on the header looks weird because the component is a mirror image: it goes on the bottom of the board.

I did the schematic and layout for the display board in Altium's Circuitmaker — I'm more used to Circuitmaker's routing than Eagle, but I'm also learning about routing in Eagle. Similar to Eagle, a component is a schematic symbol plus one or more board layouts. So I defined an IN-12B component and used that. I generated the Gerbers, and sent them off to Seeed Studio because they are far cheaper than OSHPark for larger boards. Each board is 8 1/2" x 5 1/2" (215 x 140 mm).

Next, I populated and soldered the headers.

The next step is to take one tube and stick onto it 12 1-mm pins scavenged from D-sub connectors. Then insert the result into one of the positions on the PCB. Notice that inside the tube, one of the pins is white. This is the +170VDC pin. Make sure that pin goes into the square pin 1 on the PCB.

I put the initial four in at the corners, covered with a piece of cardboard, flipped the whole thing over, and soldered them in. This helps keep the tops of the tubes at the same level. Then keep going, doing four or five at a time.

Watch a few episodes of Walking Dead or Supernatural while you do this. It helps. And before you know it, you're done!