Calculate ratio of sales document where cancellation reason for items = 0

Hi there! I have a little problem here with a single KPI. What I have are two tables (Sales Document Header VBAK, Sales Document Item VBAP). Only the VBAP includes a column with cancellation reasons (which are always the same for one sales document). The offer is still open, if the cancellation reason is null. However, I dont know how to calculate the ratio of open offers, but only for the VBAK table/ VBAK.VBELN (just for the headers, not for every item). I could only create a OLAP table with the VBAK.VBELN and the cancellation reasons by clicking on distinct values in the advanced options. I tried different PU functions to join the tables and the filter ISNULL, but did not succeed Any help is highly appreciated. Thanks! Best regards