I started at around 72/3, and ended up with 80 in the test, but my highest timed/exam conditions score was 93. My critical thinking improved more than numerical reasoning, but my critical thinking was weaker in the first place. I would say the average score increases in later papers, i.e. while 70 would secure you an interview in 2010s papers, by 2020/1/2, you are looking closer to 75 (obv. people with less get interviews, and people with more don't, but as a benchmark). So don't be too disheartened by a slightly lower score in earlier papers, but also bear in mind avg. scores/scores to get interviewed have risen.
I wouldn't bother with TSA practice books (too easy), study the guide on the tsa website in detail - it tells you how to approach different questions. Work out where you have gone wrong when you go through papers after doing them, don't just work out your score and discard them, ask teachers for help if needed.