If you insist on a rule for choosing a lead leg, the closest I’ll give you is that your stronger leg should typically be in front—but remember that your stronger leg is not necessarily your dominant leg.
 
There are two basic components to a solid split in the jerk:
 
1. Knowing and having adequately practiced and strengthened the correct split position
 
2. Balance and adequate power in the dip & drive
 
You can have the strongest, most technically perfect split position ever seen, but if you’re not getting enough power to accelerate the bar sufficiently in the drive, or not balanced through the dip & drive, you’ll never get to it.
 
Likewise, you can have the greatest dip & drive ever, but if you have a weak split position that your body wants to avoid, or simply don’t know where your feet don’t need to go, you’ll never be able to capitalize on it fully.
 
So make sure you’re focused on fixing the actual problem instead of training the part that’s already good.