Japanese to English Science Chemistry; Chem Sci/Eng Chemicals testing

各試験容器に100mしの培地を入れ，被験物質添加原液および助剤原液の添加量分を除去後，添加原液を4.0mL添加し，被験物質濃度40.0　mg/Lの試験液を調製した。

This is from a report on an algal growth inhibition test.

In the previous step, a stock solution of test substance + dispersant (助剤) + medium is prepared (= 被験物質添加原). This is used to prepare the test substance solution, as described here. Also a stock solution of dispersant + medium is prepared (= 助剤原液). This is used to prepare the dispersant control.

I am confused by 被験物質添加原液および助剤原液の添加量分を除去; Does this describe the removal of the volume of medium equivalent to the volume of test substance stock solution to be added to the medium (and the same goes for the dispersant stock solution). Or am I completely misunderstanding this?