When I was growing up my Mother and Aunts used the sheet method. Then they start using Certo (liquid form). I used that for years. Now I use sure-gel or Certo. Both very good. I do believe they would add more fruit to have it thicken with the sheet method. Been a long long time.

Quince used to be used to cause jelly to jell but I don't know the amount. vaguely remember Grandma using it(we had a quince tree out back where she used to pick them)