New
#11
That will be easy enough! First see how much is presently on the drive if you intend to shrink C way down to allow room. If you have 70gb on you wouldn't want to shrink it down to 80gb! Too small!
For a larger storage partition plus leaving a good amount of space on C after a good 120gb upto 200gb for C and over 200gb nearly 240gb+for the new D or E,F,G, etc second partition will work for a close to even split.
It mainly depends now on how large you want to keep C as well as the size of the new partition.