You clearly have not watched the free lectures that go with the lecture notes. You should not use the lecture notes on their own – it is in the lectures that I explain and expand on the notes. (If you cannot watch the lectures then you need to buy a Study Text and study from there).
We divide by 400 because the profit is always calculated as a 3 month percentage, because they are 3 month futures. (divide by 100 to get a % and divide by 4 because there are four 3-month periods in a year).
I also make clear in the lecture that in Paper F9, although you are expected to understand how futures work, you cannot be asked calculations on them. The example is there simply to make it more clear how they work.