The same question twice

I get the feeling that I'll be asked about working on Friday again, which has happened quite frequently. This time there's no overtime pay since it's going to be a shorter work week on account of the holiday. If I just assume that I naturally work Fridays now and by default have a 49 hour work week, the question I'd be asking myself is am I willing to give up 9 hours of pay to have someone else cover the shift for me. However if I assume that I work 40 hours by default and do not normally work Fridays, the question I ask myself is how much money will make it worth it for me to come in on Friday. This should be asking the same thing and I should wind up with the same result both times, either I work Friday in both situations or I do not work Friday in both situations. In practise, it doesn't quite seem to work that way though. I'm not sure I really want to or not next week when there's no overtime pay and thinking about the decision this way just makes things more confusing.