You may read this article published in Journal of Antimicrobial Chemotherapy (2003) 51, 373–378.
Recent reports have mentioned that ratio of 4:1 may also be efficacious and som reports also mention 8:1. However the time tested dose of 2:1 has been the standard.

The aims of this investigation were to calculate the pharmacokinetic parameters of amoxicillin
and clavulanic acid, and to identify parameters that may affect their observed differences in
absorption. Data were obtained from plasma concentration–time curves from four different
open, randomized, two-treatment, two-period, two-sequence, crossover Phase I bioequivalence
studies, with the following co-amoxiclav formulations: tablets 250/125, 500/125 and 875/125 mg,
or 10 mL of an oral suspension 250/62.5 mg per 5 mL. Data from 144 subjects and 288 drug administrations
were available for evaluation. After a 125 mg clavulanic acid dose (administered as
potassium clavulanate) for all four different formulations, the clavulanic acid AUCt data ranged
from 1.5 to 8 mg·h/L, varying by a factor of 5. The absorption of clavulanic acid was not related to
the absorption of amoxicillin, or demographic factors, and we were unable to identify the
reasons for the large variability in the absorption of clavulanic acid. We conclude that the
absorption of clavulanic acid, after oral administration, is highly variable and may vary over a
five-fold range between patients.