This article proposes an approach to determine the loads to apply during down-scaled testing of wind turbine (WT) pitch bearings. Down-scaled testing consists in using already existing smaller test-rigs as part of the design and validation of new larger bearings. It allows to accelerate the development of WT pitch bearings and reduce the testing costs. It also poses the problem of which loads to apply on the down-scaled bearing in order to reproduce the fatigue life and defect formation of the larger reference bearing. The goal of this work is that down-scaled testing replicates strains at key locations around a failure mode of interest, so that the testing outcomes are representative of the operation of the reference bearing. To this end, an Augmented Kalman Filter (AKF) is re-purposed, with as measurements the reference strains to replicate and as model the model of the scaled bearing. As a result, the AKF estimates a load which reproduces the reference strains when applied to the scaled bearing. This is demonstrated numerically on the use-case of testing a 3.4MW WT pitch bearing for ring fracture at an outer ring bolt hole using a 1.5MW WT pitch bearing test-rig.