Here is Some of the custom figures I did in the past few years & the ways I make parts for 3D Printing..
Parting is really important Once your client confirm your figures. Carefully Adding thickness will help your figures to print out well & Parting different area like Pant, Shirt & head or hand will help your Painting process a lot after successful Printing.
I Hope my partings for these figures will help in your 3d Printing!!
Happy 3D_Printing!!