The majority of folks that I know who did not pass the NAPEX had a difficult time with calculations (including biostats, pharmacokinetics, and insulin conversions/initiations like correction factor or splitting between 70/30s to long + rapid acting products).
For RxPrep, the calculations portion for me was pretty spot on - did you utilize the 100-question NAPLEX questionnaire test on NABP portal? Did you use both the book + test banks online? What did you score last time you took the exam and did you run out of time?
Here is how I studied in preparation for the exam (using RxPrep book + online test banks):
2022 NAPLEX - My Own Experience
** For additional math problems, look here
--> SDN Calculation 120 questions
Unless you can get >99% on all calculations and feeling comfortable on memorizing specific equations/conversions, I would not suggest anyone to go in and take the exam. All questions are not treated equal with more points/emphasis on math problems. Once you've done that, I would highly suggest to go back to "cardiovascular anything" and get to feeling comfortable with first-line drug agents and common DDIs/lab parameters. I'd start here:
1) Hypertension
2) Heart Failure
3) Stroke & Arrythmias
4) Dyslipidemia / CAD
5) Anticoagulation
If you can master these portions, then you'll be off to a strong start (depending on when you plan to sit for the exam again). I cannot speak on any other books/websites other than to say I'd reconsider how you learn and what price you're willing to pay to get ahead start on the next exam. I'd get in contact with your school +/- tutors to better see how you best learn.
If you have specific questions - just let us know!