The common way to determine the fretboard shape is to make a drawing on a large sheet of paper. Here is annother approach with the help of a calculator or a spreadsheet program.
Two measurements determine the width of the fretboard: The nut width X and Y the string spacing at the saddle (B) plus two times d ("d" is the distance from the outer strings to the fingerboard edge which should be equal along the entire fretboard length).
You can calculate the width of the fretboard at any length (L):
L ( Y - X )
WIDTH = X + -----------
S
d ... distance of outer strings
to fingerboard edge
X ... nut width (A + 2d)
Y ... bridge stringspread (B) + 2d
L ... distance from nut
S ... scale length
With the formula shown above you can for instance calculate the width at the 23rd fret (the end of the fretboard on a 22-frets guitar) which gives you all the necessary data for drawing the fingerboard outline:
For instance:
Width at nut (X) = 42mm (1-5/8")
Distance nut to 23rd fret (L) = 462.14mm (18.195")
d = 3.5mm (1/8")
String spread at the bridge (B) = 54mm (2-1/8")
Y = B + 2d
Width at 23rd fret = 55.97mm (2.203")
There's a fretboard shape calculator available