Calculation script 

If you write and use your own calculation scripts, then you should always use only the price variables. Use the price variables and the TAXpercentage and TAXincluded constants. You can use the local PHP variables $calculatedArray and

use TYPO3\CMS\Core\Utility\GeneralUtility;
use JambageCom\TtProducts\Api\BasketApi;

$basketObj = GeneralUtility::makeInstance(Basket::class);
$itemArray = $basketObj->getItemArray();

to make your own price calculations.

10.calculationScript = EXT:my_extension/Classes/Utility/MyCalculations.php