US 869 OR RETUMBO

Looking for some advice here on these two powders.I have been using retumbo for a year with good results in my 30-378 but now im going to a 30" barrel instead of the factory 26"barrel an i wondering if going to the slower powder would be a better choice. So if anyone has some ideas an some starting points please let me in on them. Im going to be using 2 different bullets to. One of them is a 175 grain vld berger an the second is the 210 grain vld berger,an the barrel is a 1 in 10 twist.Thanks