I found that the most reliable method is to weigh the cooked pureed apples. This recipe generally makes around 800 ml of apple puree that sets just the way I want with 53 g of custard. To find out how much of the custard you need is simple if you weight your apple puree and divide the number by 15 (this number represents millilitres per 1 gram of custard so you need  1 gram of custard for every 15 millilitres of apple puree). Example: 800 (the total amount of apple puree in millilitres) : 15 = 53.33 (grams of custard) ⇒This means that if you divide the total amount of puree with number 15, you get the right amount of custard in grams that will thicken this dessert.  Not enough custard and the apples will not set, too much and it will taste too rubbery.