I believe your questions are:
#1
Is the minimum load listed in the load book for a particular powder as low as you can go or can you safely go lower than the minimun and thereby reduce velocity? Are some powers safe for doing that and others not?
and:
#2
I would like to use the same powder for the magnum load and then the reduced velocity one as well.
With regard to the former: Rumor has it that if you go below minimum listed loads you run the risk of the powder detonating. Now mind you, I've never heard a first hand account of that happening,
but why take the chance. Manual publishers think it' important enough to state in their manuals.
Concerning the latter: I have one "magnum." A 300 Winnie. The reason I have it isn't the question here, but I don't need, or want, magnum velocities!
So, when developing a hunting load for use here in Jawja, and remembering
NOT to go
BELOW the minimum published load, I used the minimum loads for all the powders I tried. The load I selected clocked out right at the published velocity for the 30/06 with the bullet weight I was using. Close enough to what you are wanting to not make much of a difference.
If you want to reduce your velocity even further, look carefully and you might find a powder that works well in reduced loads. Speer used to publish reduced loads in their manuals and I'm sure there are other references. But if the only reason you are doing so is to find a powder you can also use for Mag. level loads don't waste your time. I'll not go into the reasoning for that statement, take my word for it... or don't.
What you want, from your questions anyway, is right there in the published loads. Develop your load and try to find a powder that gives you what you want at both the low and high end.
Sounds like great fun! I think I'll do that with mine!
Oops! I see that BBF posted while I was formulating my brief... Okay, what he said!