In reality, you can set the strings to a radius that's slightly flatter than the board's. After all, the strings are at least 0.25 inches above the board at the bridge, so you could set the strings to a 7.5-inch radius to match a 7.25-inch board or 9.75 inches to match a 9.5-inch board.
How important is string radius?
A guitar radius gauge is used to compare the string's height with respect to the fretboard radius. To optimize your guitar's playability, it is recommended that the string heights match the fretboard's radius. ... Use a ruler graduated in 32nds or 64ths of an inch to measure the height of the top and bottom E strings.
What is Understring radius gauges for?
Each of these unique gauges has an accurately radiused arc on both the top and bottom surfaces, and is shaped to fit easily under the strings. Use the bottom curvature to determine your fretboard radius, and the matching top curvature becomes a reference for setting string saddle heights for best playability.