I put a couple of pencil holes along the CL by twisting my pencil along the line. I sometimes have to mark it a few times during the shaping process. On the bottom, I lay a line of 1/4 inch tape down one side of the CL and use that to mark fin locations. Sounds like a pain but so is shaving a stringer. Under 6-0 and about 2.75 inches thick, I don't use stringers. I do the rail channel corrugation thing, and lately using the carbon fusion tape from GraphiteMaster. And we don't have waves of consequence over here.
all the best
"Let the day's own troubles be sufficient for the day". - JC
Keeping a clear centerline can be a bit of a pain, but it's not that bad.
On the bottom, you mainly need it for templating/cutting the planshape and at the end for setting fins. You don't need it for shaping rocker, and with EPS most people handle thicknessing and foiling in the deck.
Deckside, it's not really necessary to have a centerline.
It sounded to me like the OP is a first time shaper who scored a free blank to play with, so I was trying to save him the drama of splitting a blank and gluing in a stringer. Trying to keep it simple.
Personally, I like the idea of a glue line and plan to do that with the foam I still have before I hotwire another blank.
Is the blank a rectangular slab of EPS with rocker, flat without rocker, or a pressure molded blank that looks like a fat surfboard?
What tools will you be using, a planer, surform, sanding blocks?
I've shaped stringerless EPS with just a surform and sanding blocks made from 2x4 and sanding belts. I even did a couple using a large box instead of a stand.
Softer EPS tends to be somewhat floppy, so you need to have decent lighting to make sure you stay on track.
Once you have the blank bottom the way you want, get the top close to the thickness you want. Use a long straight stick to make sure your rocker is where it should be.
You can make a full template then you don't have to be worried about the center line. Just keep the template straight based on your rocker.
George Greenough showed Tom Curren how to make a full template with a large piece of paper folded in half. For one offs I do the same. You can stand on it a get a good feel for what it will look like under your feet. It keeps both sides symmetrical too. Once the outline is cut and you have your thickness profile set, draw lot of lines along the rail, the deck and the bottom. I make a line at 1/2" on the bottom and on the rail along the bottom. Then I'll do a line at 1" and 2" along the rail. I'll do lines along the top at 1" intervals up to 4". These are my guides for the rail bands and they also show me the thickness profile. If you keep it a little thicker than you want it to end up, you'll have enough to cut to the lines and they'll go away as you shape.
I do a lot of stringerless boards. For strength, I make sure the rails have a lot of glass. Do double layer wraps that go at least an inch around. You can shape a slight beveled edge (like the Stewart hydrohull) on the bottom side to make up for the extra thickness of the glass.
I have EPS boards with lots of glass that are still reasonably light. Very light compared to my Poly boards, but those don't have extra light foam.
Here's a picture of the box I used to shape my first board or 2. I had to do it on my knees too.
Sorry McDing if I was unclear. When I do a board I follow a step-by-step process. Assuming I have a good template pattern to work from, I would just use the pencil line (stringerless/no glue-line blank) as a reference in transferring the outline. Once it's laid out on the blank I don't use the template pattern any more on that shape... it gets cut, squared off, and all the rest without the template pattern. I try to true the outline cut right to the line, square it off, and not get too carried away scrubbing the rail edges. Ideally, the rail tuck and rail crown meet at the apex and require just a touch of the screen to finish it off.