Seating depth is determined by the load data. Hodgdon has data for a 150gr Sierra Flat Point with H335. Plated bullets don't load the same as Jacketed. It is recommended to use LOW to MID Range Jacketed data. May want to drop a line to Xtreme and see what they recommend for top speeds in their 150gr FP. See if it can withstand even the lowest speeds and pressures.