Answer
Combine a Custom Event on the relevant logical unit (e.g. triggered on insert/update of the line record) with an Online SQL Event Action. In the Online SQL action, check the condition in PL/SQL and raise a client-visible error with Error_Sys.Record_General(lu_name_, 'ERRORIDENTIFIER: Your message text :P1', param1_) when the condition is violated — the :P1-style placeholders let you substitute actual values (like the required multiple) into the message. This does require PL/SQL and familiarity with IFS's business-logic error-raising conventions, so it's a developer-level configuration rather than a pure end-user setting.