Practically how does one use a scale if they want to eat the Matzah they use at the Seder?
Do they weigh the entire Matzah then figure out what percentage of it they have to eat based on its total weight?
That's one way. (alternatively: weigh. groan)
When I've done this for a big family, I portioned out kezeisim in plastic bags, labeled with each family member's name and the mitzva it was for.
For example, I knew that Yaakov wanted whole wheat, so I put two kezeisim in a plastic bag and marked it "Mozti Matza". Then I put one kezayis in a bag and marked it "koreich," and another kezayis in a third bag and marked it "afikomon." I then put all three bags into a bigger bag labeled "Yaakov".
At the seder, I would hand the bags out to everyone before washing, and everyone would have their matzo prepared for all three mitzvos. All they would need is a small piece from the leader of the seder, and they already had their preferential type of matzo portioned out for them.
It saved tons of time, and it saved tons of matzo, as everyone always overestimates kezeisim.