Converting a long nested if and or formula into vba case

I have a nestled If and or formula that I am trying to convert into probably a Case formula using VBA (or any other suggestion would be great), but I am a beginner and not sure how. The reason is that this formula currently is in every cell AG12:ACG500 and takes up so much memory that the spreadsheet is extremely slow.

Basically, I am trying to match the date in column Z12:Z500 (Outage Month Start) to the date in Row AG6:ACG6 (DATES), then looking in column C12:C500 (ACTV_NAME)... which provides the output of either "R, S, L, MR, MS, ML, ?R, ?S or ?L" in the cell where the two dates (column Z and row 6) intersect .. this should coincide with the where the Gantt Chart bar starts .. I do not need any help with the Gantt chart bars/color coding .. I just need help basically labeling them with the fore-mentioned.

"=IF(OR($Z12="",$AA12=""),"",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Relay",$L12="No"),"R",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Substation",$L12="No"),"S",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Line",$L12="No"),"L",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Relay",$L12="YES"),"MR",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Substation",$L12="YES"),"MS",IF(AND($AF$6=$Z12,$Z12<>"",$C12="Line",$L12="YES"),"ML",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Relay",$L12="Maybe"),"?R",IF(AND(AG$6=$Z12,$Z12<>"",$C12="Substation",$L12="Maybe"),"?S",IF(AND($AF$6=$Z12,$Z12<>"",$C12="Line",$L12="Maybe"),"?L",""))))))))))"