The menu is your list. A function is a name and a formula written around {cell} — the cell's own content — so one entry covers every cell you ever point it at.

How to Use Add Function

Click My Functions, then Add Function.
Give it a Name — that is what the menu will show.
Write the Formula around {cell}: ={cell}/{divisor}, =ROUND({cell},{decimals}), or both nested at once, =ROUND({cell}/{divisor},{decimals}).
Watch the two lines below. Example shows what =A1+A2 would become, brackets and all; Asks for lists the parameters, or says nothing — applies at once.
Click OK. The function joins the end of the menu immediately — no restart.

Any {name} other than {cell} is a parameter, asked for each time you apply the function and remembered afterwards. Type a number, text in quotes, or a reference such as $B$1 to keep the value live.

To change one you already have, Ctrl+click it in the menu: the same dialog opens with its name and formula filled in, and a Remove button beside OK.

Note: Formulas are written in Excel's invariant syntax — English function names, a comma between arguments — so a list keeps working whatever your Windows locale is. A parameter you type as 0,5 is converted for you. The formula is not checked when you save it: Excel judges it the first time you apply it, and the message names the function to Ctrl+click. The list holds up to 20 functions.

My Functions