A bit too unorthodox to answer my own question, but I just found the reason for the 'warnings' and I've cleared them. So for anyone else coming here with a similar warning, here's the answer.
That compute line was just too complex for COBOL85 compiler to handle. I split up the equation into small bits and then put them together and they worked like a charm!
So there goes! No data type issues, or anything at all, actually! Thanks for all those who tried to help, btw. Much appreciated! :)