Creating Chest and shift clicking

I am having a problem with the chests I can make a block and gui that is fine but I can't shift click more then 9 times I changed this to 

 

if (index < 27) { if (!this.mergeItemStack(itemstack1, 27, (45 - 9), true)) {          return null;   // fixes shiftclick } } else if (!this.mergeItemStack(itemstack1, 0, 27, false)) { return null; }

And I was able get it to shit click from player to chest fine but now I can't get more then 9 back out of the chest I have changed the 45 - 9 to 45 -27 but this brakes it so I think the maths here are wrong anyone got any idea on this part?