I can't give you useful tips currently because I am not sure what score my boys will get from the real tests. I will try to give you my views.
For Atom learning, one boy of mine is better, he always gets 57 out of 60 for Maths, however, the score Atom gives was always different, sometimes, 117, sometimes 120. Based on this experience, in my opinion, if the difficulty level in the real test is same as Atom, whether my boy succeed or not is really random and depending on that day. For example, he was happy and may get 58 but he may get 55 if he is not happy. 55 vs 58 might not be very significant different, but the standard score is very different.
then it goes to the 'bar' set by Atom. The bar was set by Atom based on the previous boys but not very accurate. For example, a boy always get 55 in Atom but that day, he got 58 in the real test. then, the bar will be based on his experience.
I am really not sure whether the real test is easier or difficult. for my 2 sons: one said easier but he is always a little bit of overconfident and he only get 105-110 in the mock test. Yesterday, I asked the other one (who always get 57 out of 60), he said this test was harder. he can maximally get 55 correct this time.