This is such a common question for me. When I started using Gelatine and making up my own recipes I went through so many unset batches that I just wanted to quit using it. Then I looked online and began to understand about the acidity and alkalinity of various foods such as fruits and alcohols. Let me just say making Strawberry Champagne jellies needs a 2 extra Gelatine leaves.
So basically, this means that the higher the acidity of foods, such as oranges, yuzu, and strawberries, the more Gelatine you need. When developing your own recipe and have these ingredients in the mix, you are going to have to experiment with the amount of Gelatine you need.
One way I have found that works for me are that for every 1 leaf I use I add a half leaf (1:1/2 ratio). But depending on the amount of fruit or other inclusions you may need to add the Gelatine in 1:1 ratio.
Having told you about how to set the Gelatine you also need to be aware that there are some foods that stop the setting process altogether. Kiwifruit, pineapple, paw-paw, mango, and peaches have an enzyme in them (as well as being highly acidic) that break down the gelling properties.
There is a way to use these fruits, you need to stop the enzyme (Bromelain in pineapples) from working. To stop this enzyme, you need to heat the fruit, basically, cook it. You can also use a canned version of the fruit. Don’t let this put you off from making a magical fresh pineapple and Kiwi fruit jelly, you can substitute Agar (Agar-Agar) for the Gelatine and make a wonderful creation that rivals Gelatine based desserts.
To buy gelatine, head over to Gourmet Goldmine.