Every piston will be coming down over at least two stacks of blocks. The piston will shatter the weakest blocks in the stacks if they add up to less than the piston force.
Your goal is to have the weakest blocks in the stacks under a piston total more strength than the force of the piston coming down on them. So, a piston decending with a force of 7 will shatter the weakest blocks in the stacks under it if the weakest blocks add up to less than 7. But, if they are greater, that piston will be stopped! So if there are two stacks under a 7 piston, and the weakest block in each stack is 4, then the total is 8, and the piston will stop.
