Don't forget that boards don't only shrink when they dry, they also expand when they're wet. The wider the board, the more it will move.
I think that trying to set the gap with something referencing each board as you go along is almost guaranteed to result in wobbles. The best way to do it, (yeah yeah, broken record) is to lay every fourth or fifth board to a chalk line and then fill in between them, using wedges to set the gap. That way you are guaranteed to end up with straight lines. With your method, if one board is a bit fat at one end, or it springs back after you nail it, you can end up with an error that compounds itself as you lay successive boards. If I can convince one person on here to give it a try then I will consider my job done.
"I don't practice what I preach because I'm not the kind of person I'm preaching to."