Hello Adrien
About TAX alculation scheme
https://scn.sap.com/thread/2142201
http://help.sap.com/saphelp_srm30/helpdata/en/0f/5c0a3b79415375e10000000a11402f/content.htm
About TAX Calculation Procedure
http://help.sap.com/saphelp_46c/helpdata/en/e5/077fc44acd11d182b90000e829fbfe/content.htm
Hope helpful & Kind Regards.