.031 may be a little small (that is the standard size hole for an IC leg) from what I remember of nixie tubes (I haven't seen one in 15 or 20 years) the pins are more likely to be around .1 post size or .038 in I think. You probably want to measure a pin with vernier calipers before committing to a board I expect. In Inkscape hole size is pad dianeter less stroke-length*2, so for a .038 pad with the normal 20 strokw width the pad diameter needs to be .078 in (which when you subtract 40 for the stroke with leaves you with .038 for the hole size). Your board looks to be in something like mm but the calculation should be the same.
Peter