SUM, Percent

How to find the sum of two measures ? Eg : Created two fields in beast mode-> Old G/L =  CASE  'Age in month' > 12 then Old

 Current G/L =  CASE 'age in Month' >3 then Current .

There fore,  want to find the Total Entries= Old G/L +Current G/L

secondly want to calculate  percent of  Old Entries = Count (Old G/L)/TotalEntries * 100