Spin-constraint precision ﹩{\sigma }_{| {a}_{* }| }﹩ for M87* with prograde (blue region) and retrograde (pink region) accretion flows as a function of relative astrometric error σ_r.a.. In the darker regions, the n  =  1 ring diameter ﹩{\hat{d}}_{1}﹩ is taken to be coincident with that of the n  =  0 ring and the spread in ﹩{\sigma }_{| {a}_{* }| }﹩ accounts only for variation over all spin magnitudes 0 ≤ ∣a_*∣ ≤ 1. In the lighter regions, the n  =  1 diameter is taken to lie anywhere within the n  =  0 ring. For comparison, the standard deviation corresponding to a flat distribution in spin is also shown (black dashed line).