Retumbo is really outside the window of ideal burn rates for the 270Wby's case capacity to bore ratio. Better choices are IMR7828ssc, H1000, RL19, Accurate MagPro, and Ramshot Magnum.
Unfortunately, unless you have some older manuals, the data currently available for most Weatherby cartridges, including the 270Wby, is dismal, at best. Consider the Nosler and Barnes data the only remotely realistic data available currently.