Page fraction — <math>Frac(page, UB)</math> — normally should be <math>\frac{1}{N}</math>, where <math>N</math> is the number of UBs the pages are shared between, but this is bad since adding a new UB to page shared set would require recalculation of the whole current set. In UB <math>Frac(page, UB) = \frac{1}{2^{UB_{shift}}}</math>, where <math>UB_{shift}</math> is some parameter which [[RSS fractions accounting|is calculated ]] so that