To achieve this, the improvement rate of surrogate quality needs to be quantified. MSE can be used to quantify the regression performance. We use the moving mean (89: ;;;;;; ) to smooth the fluctuation of the MSE curve. The improvement rate of surrogates can be defined as the MSE decrease per data added, or we call it the <=>?@ &'( as Equation (1). A scaling function can scale the slope values to a relative value, A@=BCDE@ <=>?@ as Equation ( ). This value is generally between -1 and 1 (due to the fluctuation, the value of the slope can be positive). The added ratio function can convert the relative slope to the value of the added ratio (typically between 0 and 1). Following this, the number of added samples is calculated by Equation ( ), where 7 )**%+ is the maximum adding number.