Spacing wasn't perfectly even but on average about 100 mm apart.
With plugs & feathers it's advisable for the holes to be in a line, to be considerably deeper than the length of the plugs & feathers - and to be as near as possible perpendicular to the rock surface. That's in an ideal world but sometimes, in a boulder with an irregular surface, it's not always easy. Done properly, it's quite amazing how powerful these things are.
The stonemason I had the good fortune to bump into t'other day was describing their use to split off rectangular blocks as long as 30 m, very precisely. There are videos of professional use of plugs and feathers, on a scale larger than we potholers would ever be involved with, on You Tube.