Iam shaping a board from scratch. I'm using hand tools. The rails have me scratching my head .

my intent is not hard, not round but 80/20. I have room to work as iam not at my widths yet.
how can I maintain a consistent shape, and should I alter it toward the rear?
its a FSW shape 105liters plan, generally like a Simmer Quantum, Tri fin.
At this point some of the blank looks to be FAT, but I'm afraid to remove too much, IM using a Skate as thinckness guide, by a measure .
Shape and rail suggestions, most welcome . Other pick on me remarks ok, I have thick skin sometimes.