I have a site provided by an APUGer in which you simply put in current exposure time and enlarger height plus new enlarger height and a hidden excel prog gives you the new time.
I cannot give you the URL for it as nothing shows up. I simply saved it from whatever was given on the post. Sorry to be so vague. Someone might refer to it here or the original poster will respond but if not do a search on enlarger posts and it should come up.