the reference is
Meeten, G H and Lebreton, C (1993) A filtration model of the capillary suction time method Journal of Petroleum Science and Engineering Vol 9 No 2 pp 155-162
CSTcircular = [π Є h / S Af]2 (1)
where CSTcircular is the circular capillary suction time,
Є is the porosity of the filter paper,
h is its thickness,
S is the desorptivity of the suspension at the capillary suction pressure of the filter paper and
Af is the area where filtration takes place.
In most cases using the same sludge you can assume Є, h, S are constant so the equation becomes:
CSTcircular proportional to [π/Af]2