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.