Jeff,
Here is where I am comming from.....my Match gun/hunting gun...300 wby, 30" spencer 1-10 twist, Bat action single shot "M". 88 gr Retumbo, nothing. 89 gr Retumbo, slight cratering of the primer. 90 gr. little more primer flow, and a little hitch at the top of the bolt throw uppon ejection. Back down to 88 gr again. Shoots lights out!!! Now....my velocity with 88g Retumbo is 3275 FPS....with 210VLD's out of a 300 WBY.
Is that load ...."OK"..... Hell no....the chrono tells me I am so far into the "red" zone it's not even funny. I am not sure where "max book" is with a 210...probaly around 83 gr of Retumbo. And that is usualy with a Weatherby freebore, not a tight neck chamber with the bullet jammed hard. I, like most of you, am not a big proponent of max book. I am north of max book most of the time. But, they come up with max book via PSI numbers. That has to mean something.
BUT.....BUT.....again...BUT.....I still shoot that load. I have never tried it in warmer weather than I tested...around 30 F, and I am never going to.
My point is.....just becuse everything seems ok, the fact that i am getting the velocitys that I am getting tells me that this load is not REALLY ok. If you are one grain away from overpressure with retumbo in 30 378, which would be in the 100Gr neighberhood, I still say you are not OK backing off only one grain.
Again, just my .02.