Welcome to the forum. Since you didn't indicate otherwise, I will assume you are new to reloading. Therefore, I recommend that you not stray from the loading manuals.
I have never found a reason to load magnum cases with sub-velocity loads. Keeping track of different case sizes seems easier than keeping track of light magnum loads. Also, small powder amounts in large cases is inviting inconsistency due to the vector angle of the fire arm. Of course, you could use inert ingrediants (such as corn starch) to hold the propellant against the primer, but that would be deviating from the loading manual.
Good luck and welcome.