Like everything else that’s useful in life and not taught in schools, the subject required here is found in the Hitchhiker’s Guide to the Galaxy. It’s called Bistromathics, the first principle of which is that of nonabsoluteness:

The first nonabsolute number is the number of people for whom the table is reserved. This will vary during the course of the first three telephone calls to the restaurant, and then bear no apparent relation to the number of people who actually turn up, or to the number of people who subsequently join them after the show/match/party/gig, or to the number of people who leave when they see who else has turned up.

The second nonabsolute number is the given time of arrival, which is now known to be one of those most bizarre of mathematical concepts, a recipriversexclusion, a number whose existence can only be defined as being anything other than itself. In other words, the given time of arrival is the one moment of time at which it is impossible that any member of the party will arrive. Recipriversexclusions now play a vital part in many branches of maths, including statistics and accountancy and also form the basic equations used to engineer the Somebody Else’s Problem field.

The third and most mysterious piece of nonabsoluteness of all lies in the relationship between the number of items on the bill, the cost of each item, the number of people at the table and what they are each prepared to pay for. (The number of people who have actually brought any money is only a subphenomenon in this field.)

Simply apply this principle to your paycheck, taking into account that there is a drunk version of you ordering random shit you don’t need on Amazon or getting Taco Bell delivered at 1am, and off you go.