Thanks for reaching out.
McKinsey PST and BCG Potential test are quite different.
Indeed, for BCG Potential Test, you are allowed to use a calculator and the difficulty lies in three main things:
1. the point system: +2 right answer, 0 no answer, -1 wrong answer. The idea here is not to gamble too much when you answer your questions.
2. Being fast and accurate: BCG Potential test gives you two sets of informations at first (with introduction, quants and testimonies from executives in the firm) and then you have to adress one or two problems. The idea here is to navigate between those informations and look for the right one.
3. Qualitative questions: Here the idea is to look closely to the options (give a recommandation to the CEO type) and eliminate irrelevant answers.
For BCG Test, there are some examples that are free in the internet.
For McKinsey PST, the idea is to use math tricks in order to increase your speed and accuracy.
For McKinsey PST, I advise you to work on the examples on McKinsey website as well as old PSTs. I Got An offer has also a pack to prepare for PST (But I don't want to do marketing for them here :) )
Finally, to increase your numerical accuracy (which will be also helpful for the cases), I advise you to work on Math apps/GMAT.
Best of luck and do not hesitate to contact me if you need any help.