Find a data calculation error in the yii2 app

There is an application on yii2 that performs mathematical calculations. One of the calculations does not work correctly, there is an error in the calculation of data, I have a formula not difficult to calculate, you need to find the reason without breaking the functionality, the application has a lot of dependencies, so nothing extra to touch.

App description https://docs.google.com/document/d/1icoWBrpWCDy_RY...

2 tasks and a description of what needs to be done https://docs.google.com/document/d/10EmslFy7LAh93j...

Payment in any way upon completion, there is a git.

Requires a person with a confident experience in php.

At the end of the new list of tasks, with a budget of about 40,000 rubles for the next 2 weeks.