As re the stringerless lack of stringer for reference, when I traced my templates, I did it with a blue highlighter marker, all the way around, including the center “stringer.” It’s bright, doesn’t leave a groove, and sands off easily.
I guess what I’m saying there is that when shaping the bottom rocker, I would continually plane or sand of my reference points and need to create another to check my work.
I was just up early reading this old thread…I shaped a ton of sailboards throughout the 80’s both in Clark Foam and EPS ranging from 2 lb. to the ultralight .5 lb (basically just forming air). Sailboard design was the early forerunner to all the concave stuff found later in surfboards. By '82 or '83 I was shaping single to doubles, tri-caves, quad caves, or other combo’s w/planing rails, etc. on a daily basis. A lot of this stuff, I crossed over into surfboards (all the concaves, square vertical tail rails, extreme contoured decks, basted layups for super defined edges, and on and on.
What I found for distinguishing concave ridges or vee, or any type of separation was the use of strategically laid masking tape. This worked well in the sanding block stage of finishing out a shape so you could maintain a distinct line on the shape. Hope this helps you out in the future.
I cut my own rocker, but I see your problem. I recommend that. Probably if it was me, and I’m riding small mushy waves without a lot of vertical yearnings (oh, wait, that is me,) I’d leave it flat and just get the fins and rocker and rails right, eyeball it, and maybe one day I’ll make some calipers. And I think about how Griffin leaves it flat for Hawaiian waves. This is just a peanut’s opinion, my 2 cents, but I’m coming to the conclusion that both positive and negative bottom contours, rocker too, suck a lot of time. Between separation and suction…