Dome Calculator Spreadsheet Question

[pmguy1981]

I'm curious to what the Inside bottom and the outside bottom columns are referring to. I'm new at this and honestly have no clue what these columns mean.

[Saluki]

I'm guessing that the width of the bottom side of the brick, thats the widest point. Its tapered and beveled according to the chart so the bricks progressively get smaller as the radius shrinks.