The module itself is very old and proven. And separately it has nothing to do with calculations.
There were lots of changes in the rounding in thirty bees.
In the most current version (1.6) it has no differences so what I can advise is to research gradual update to 1.3, then 1.4, 1.5 and 1.6.
During those updates you will notice lot's more improvements, bugfixes, security fixes and new features.
In order to do the updates you will have to install Collect logs module and update your Core updater module.
Look for each version release thread here https://forum.thirtybees.com/forum/2-announcements-about-thirty-bees/ and follow the update instructions to each version.
Generally no or very little modifications are needed to the theme (if you are using 3rd party one) and the majority of modules continue to work.
Running more than 5 years old software is not recommended.