I did a search on www.google.com with the phrase "intel hex file specification" and got a lot of hits. One link below takes you to a site from where you may look at the original Intel document that defined the hex file format. This is dated "Revision A, January 6, 1988".

http://www.filespecs.com/formats/e...ontent.jsp

Within this document you will find out everything you want to know about how the checksum is calculated.

Have Fun
Mike