we would normally go for half the tol as thumb, but if roundness is being measured then its not just the tolerance but the shape of the roundness which is also important which can highlight issues, especially if the hole as a keyway in it, in which case the roundness can be looked at locally away from the keyway. We would also apply half the tol rule mainly for bores/shafts that form part of an assembly as a concentricity tolerance since this is typically what comes out from bearing calcs, but if its not called for then stay away