What is the correct Marlin firmware setting for Tevo Tarantula with MKS Base V1.2 board
The MKS Base v1.2 is basically an Arduino MEGA2560 and a RAMPS1.4 on a single board.
You can use the RAMPS pin designation.
Note that the endstop to origin distances is explained in question How to center my prints on the build platform? (Re-calibrate homing offset).
In Jims Marlin fork you can find this at:
#define X_MIN_POS 0 - XTRA_BED_LEFT

and
#define Y_MIN_POS 0 - XTRA_BED_BACK

Unfortunately, XTRA_BED_LEFT and XTRA_BED_BACK are defined as zero; this implies that the endstops define the origin (not likely) or it is left as an excercise for you to find out. This answer explains how you figure that out.