Recently, the millimetre-scale, highly efficient growth of single-wall carbon nanotube (SWCNT) forests from iron (Fe) catalysts has been reported through the annealing of the magnesia (MgO) underlayer. Here, we report the modulation of the CNT yield (height) and average number of CNT walls for a Fe/MgO catalyst system through the collective effects of initial Fe amount and MgO annealing temperature. Our results revealed the existence of a well-defined region for high yield SWCNT forest growth in the domain of deposited Fe thickness and MgO annealing temperature. Through topographic examinations of the catalyst surface using atomic force microscopy, we confirmed that our results stem from the collective effects of increased amounts of surface-bound Fe through the amount of deposition and suppression of Fe subsurface diffusion, together govern the amount of surface-bound catalyst. The combination of these mechanisms determined the final nanoparticle size, density, and stability and could explain the three distinctly defined regions: low yield SWCNT growth, high yield SWCNT growth, and high yield multiwall CNT growth. Furthermore, we explained the observed borders between these three regions.