If handling this in front end then @mcsshg2011 is probably the way to go, but if you are able to manipulate the script then you have two possible solutions.
Manipulating the header by adding one month:
Data LOAD Closing,
Capitalize(Date(MonthName(date#(Subfield(Closing, ' ',1), 'MMM-YY') , 1), 'MMM-YY')) & ' No. of Emp' as Opening,
(html, codepage is 1252, embedded labels, table is @1);
Reading the previous loaded data:
By fetching the Monthly amount from the previous line read you are able to get the previous month value, but you wont get a value for 'Apr-19 No. of Emp' as it is not in your data. It is also important that you are able to sort your input data correctly, hence it is reading the previous line.
Plees ekskuse my Swenglish and or Norweglish spelling misstakes