Long-duration and efficient quantum memories for photons are key components of quantum repeater and network applications. To achieve long-duration storage in atomic systems, a short-lived optical coherence can be mapped into a long-lived spin coherence, which forms the basis for many quantum memory schemes. In this work, we present modeling and measurements of the back-and-forth, i.e. reversible, optical-to-spin conversion for an atomic frequency comb (AFC) memory. The AFC memory is implemented in ¹⁵¹Eu³⁺:Y₂SiO₅ with an applied magnetic field of 231 mT, to suppress time-domain interference effects in the conversion efficiency. By optimizing the conversion using the developed simulation tool, experimentally we achieve a total efficiency of up to 96%, including the spin echo sequence and spin dephasing, for a storage time of 500 µ s. Our methods and results pave the way for long-duration storage of single photon states in ¹⁵¹Eu³⁺:Y₂SiO₅ with high signal-to-noise, at the millisecond timescale.