depth of crack Swallow tail formula

Aloha,

i was reading greenlight design guide website about tail shape. If i understood what they say, to calculate the deep of crack for a swallow tail they multiply tip to tip distance of the tail by 0,49.

I took a template (5'4 Steve Lis) from blending curve, tip to tip distance is about 29,4cm (11,56") and deep of crack is about 5" (12,5cm).

if i calculate 11.56"x0.49= 5.66" i am far from 5"

How you guys calculate your deep of crack?

Thanks.

Eric.