how to save a function to use in the future

I'm either trying to write a vba code to save this function to recall it in the future,   =ROUND(SUM(IF($A$50:$A$2205>= B$1, IF($A$50:$A$2205<=B$2, IF($C$50:$C$2205=$A3,$B$50:$B$2205,)))/1000),1) ...