Lost's Rocket concave Help

HI !!!


I have my hands "now" on one blanck,to shape some like the Lost's Rocket model,


I have in my measures the deep point in the concave at 1/4 down,

but I don't know how long I can 'eat' foam from the stringer to the rails??,for do the "out line" of the concave in the bottom of the board..


Can anyone give me some answers??!! thank!!