B1+ field inhomogeneity is a major source of errors in quantitative mapping. The accuracy of B1 maps, depicting the effects of B1+ inhomogeneity on the flip angle, is thus critical. However, there is no gold standard B1 mapping method in vivo so absolute accuracy is difficult to determine. In this work, we propose steps that exploit known B1 effects in a small phantom to obtain absolute accuracy estimates in vivo. Two B1 mapping methods are required, but neither need be accurate. We demonstrate the proposed assessment by obtaining stability and absolute accuracy measurements of the Method of Slopes B1 maps.