Jengajen
Unless stage 1 has changed massively since I did it last year then it's hard to compare the online test with SHL as on the test day you are only timed to do 40 odd questions and not on acomputer. Therefore you will have longer to do the ones you get stuck on and then fly through the others. I passed stage 1 last year and given the time pressure in the online tests I found them more tricky than the tests at nats. With regards to calculations , I would say they are more to prepare you for what is to come. I did no such calcs last year just relatively simple dst questions. Rather than back calculating to find that bloomin " ? ".
As probably said before I would say that to learn the ATC stuff backwards. If you score ok in the maths type questions and do poorly on the motivational stuff, I guess it won't be a great result. I don't think stage 1 will be as bad as you think if you have done enough prep.
Good luck to you and all stage 1 people. May see some of you at whiteley on 19th am session