Thanks for your reply, @Minas6907, refractometers are as I thought not made for making jams. Using it while the jam is boiling is not really practical. A refractometer gives only correct readings at an ambient temperature of 20°C. They even provide that thing with a rubber handle to prevent influence from the warmth of your hand holding it. So, waiting for a drop of boiling hot jam-in-the-making to cool down first, before putting it in the refractometer, while the pot of jam is boiling away is not very practical to say the least.
As I mentioned in my earlier post, a refractometer only serves to determin the brax value after the jam is made and cooled. It has no use in finding out how much sugar to use for a given batch of fresh fruit.
I'm also not all that convinced that a jam should be taken to 75°brax. that means simply that the jam or jelly holds 75% of sugar, which is very high but not impossible if the jam cooks for hours and the sugar content gets systematically higher due to the evaporation of liquid in the jam; maybe your pâte de fruit or confit fruits go to such a high brix value?
I'm not making jams and jellies for selling, it's for personal use only. I even made "membrillo" a few times, a pâte de fruit made from quince.
Homemade jam and jelly need much more a pragmatical approach. But, rule of thumb since ages; 1 kilo net fruit and 1 kilo of sugar does the trick. And 1 kilo of juice with 1,2 kilo of sugar for (clear) jellies.
However, jam making is not a matter of sugar content alone. The jellification happens when the pectin and acid in the sugary concoction start to work together to form a type of gel. You can add all sugar you want to a strawberry jam, it won't set as long as there's no pectin added. Strawberries don't have pectin.
Also, it's always a good idea to add the acidic juice of a lemon per 1,5 - 2 kilos of net fruit to activate the natural pectin in the fruit you use.
Nowadays, imo, it's best to simply use commercially available pectin sugar. It's a sugar with added pectin and citric acid. It exist in different pectin/sugar concentrations, so reading the package helps, following the instructions to the letter is a must. The advantage of using pectin sugar, is that you can reduce the sugar content. I now always use 1000 grams (1 kilo) of net fruit for 800 grams of pectin sugar. I know people who use an even much lower pectin sugar content. Also and very important; the cooking time of jam made with pectin sugar is 4 minutes!!! You could go to 7 minutes, after that the jam may coagulate no longer! The jam sets when cooled down.
Pectin is not something artificial. Pectin is a natural product that appears in most fruits. Citrus and apples for instance have a very high pectin content while as said earlier, strawberries have almost no pectin. I don't use pectin sugar when making jellies, it makes the jelly cloudy.
And then there's of course the jams and jellies made with a sugar alternative like Chef Buba does. Pectin can be bought separately, so you can use it in combination with sugar or stevia etc.