Adding the three characteristic quark energies from b gives
(1)
The estimate recovers a sub-GeV scale but underestimates the stated neutron mass by a factor of a few. This is consistent with its deliberately crude uncertainty and geometry assumptions; it is not a precision mass prediction.
A better model needs the actual spatial and momentum correlations, a confining interaction, and the energy in the quark and gluon fields. In Quantum chromodynamics, hadron masses are not simply the sum of three freely moving particles with tiny masses. The supplied MeV rest-energy scale resembles a light current-quark scale; a phenomenological constituent-quark mass includes substantial dressing and is a different parameter. Color, spin correlations, relativistic dynamics and sea components all matter when defining a more realistic bound-state Hamiltonian operator or performing a lattice QCD calculation. The prescribed uniform-sphere model supplies none of those extra dynamical inputs, so its discrepancy should not be repaired by an unannounced change of the given radius or mass.

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