Error budget. We compare the MC derived uncertainties σ_MC associated with the BB^lens measurements to the semi-analytical errors σ_G obtained using Eq. (15). Top: averaged BB^lens band-powers using either σ_MC (blue) or σ_G (red) as error estimates. For illustration, we also show the expected sensitivity of the PlanckSMICA CMB polarization map to the BB band-powers (grey boxes), whose uncertainties are a factor of four larger than those of the BB^lens band-powers using the lensing B-mode template. Bottom: relative difference of σ_G with respect to σ_MC. Dashed lines show a 15% difference.