We examine the lamellar/cylinder and cylinder/sphere phase boundaries for strongly segregated diblock copolymer melts using self-consistent-held theory (SCFT) and the standard Gaussian chain model. Calculations are performed with and without the conventional unit-cell approximation (UCA). We find that for strongly segregated melts, the UCA simply produces a small constant shift in each of the phase boundaries. Furthermore, the boundaries are found to be linear at strong segregations when plotted versus (chi N)(-1), which allows for accurate extrapolations to chi N=infinity. Our calculations using the UCA allow direct comparisons to strong-segregation theory (SST), which is accepted as the chi N=infinity limit of SCFT. A significant discrepancy between the SST and SCFT results indicate otherwise, suggesting that the present formulation of SST is incomplete.