Welcome aboard Bonsai Nut!
Picking a pliable leader is a good thing as one can wire it up and seamlessly continue the trunk line. Would do the cut low , say 1/2 way down if you desire to develop taper.
If one just wants a chunky base, cut anywhere for now.
Be ready to repot, likely in a a bigger pot or nursery can next year or the year later. We regularly up pot redwoods.
Cheers
DSD sends