Did you search here? It must've been covered some time or other. However, the easy way is to convert the ascii to packed BCD values and then simply add the values of each such byte into a 16 bit variable. At the end of processing, you will have your LRC or checksum.

So, 032008091422 (12 bytes)
would become
03 20 08 09 14 22 (6 bytes)

Now, you add 03+20+08+09+14+22 to get the 16 bit checksum.