I find this a difficult question to answer, as it depends on storage condition of the film. I remember hearing a reference to "a stop a decade", but as well this is debated. I haven't shot slides are that expired before, but with black and white films like pan atomic x (20+ years), I would overexpose by 1.5 to 2 full stops. In my opinion, since you are shooting sheet film, and developing at home I would just shoot a test sheet, and develop at normal times. This is really one of the easiest ways to solve the problems, with good results. Also, you could develop for a half stop over to fall within the reciprocity of failure of the film, if I remember correctly most slide film fall into that window. You could do that and save time, but I feel that is a little mediocre. If you can give me the storage conditions of the film, and the conditions shot in, I feel I could help you more.