if you wish to keep the core at 1/4 and are only conserned with stiffness there are a few guidelines that you may find useful.
bear in mind that stiffness and strength are closely related but definitely not equal.
so, it's possible to make your board extremely stiff, only to have it snap like a twig ( a brittle twig
for simple beams ( boards) : stiffness is proportional to E*I / L^3
.....more or less of the above will alter the stiffness accordingly
E= elastic modulus of the material used for reinforcement
I = moment of inertia of cross-section of board and is dominated by the boards thickness
I= (b * h^3)/12 ( for a rectangular x-section)
b= board width ( a const for this example)
h= board thickness (our variable of interest and a huge factor)
L= board length ( another big driver but i'm assuming a constant in this example)
The modulus of your core certainly is a factor, especially when comparing cedar to any of the foam cores used. cedar planks will even be a lot stiffer than any plywood of similar thickness due to amount of grain aligned with the boards long axis.
but as the board gets thinner, the h^3 term dominates (or fades to nothing, in this case )
and the core material is less of a factor compared to the extra glass you'll need to compensate for reduced x-section geometry.
one way you could increase I, is by adding concave to the board (artificial thickness)
personally i think 1/4 inch planks are ideal for making 1/2 inch boards.
rocker and concave become very easy on the table once you have 2 layers for laminating.
but, back to the beginning:
if everything else is equal and you are comparing the required glass thickness to maintain stiffness as you reduce overall core thickness , i think it would all boil down to this....
(c+g)^3 - c^3 =const
g = glass thickness ( assuming the same flavor again) 6 oz is approx .010" thick
(c1 + g1)^3 - c1^3 = (c2 + g2)^3 - c2^3
if you are more like me, trial and error is an easy path in this case.
take your best guess and glass yourboard.
let it cure and compare to the board or numbers that you'd like to hit.
if the results are too soft for your taste a light sanding and another glass lam will quickly make you the expert in this matter. with no real penality for the extra steps.
if you want to make it more like a science project, a little bit of math would let you scale smaller sample sections to get all of this before committing to the full scale version, as well.
i know this is mostly a long winded non answer.
but i hope it helps and let me know if you need something more concrete.