Assistence to crack a crc-checksum algorithm.

I have a configuration file that I wish to modify and reload. The data appears to be in 235(EB) bytes chunks. Byte 0-20 appears to be a record label Byte 21 is a space Byte 22-25 appears to be a checksum Byte 26 is a space Byte 27-235 appears to be the configuration data for the record. I wand to be able to modify the configuration data and then recreate the checksum. I think there are sufficient examples in the txt file to work out the algorithm. I am playing around on a home project, so this has no corporate funding. I am hoping for fixed price project at $US 50.00, but if you are confident you can return a result for another fixed price, please place a bid. I am new to finding a freeelancer, you have to start somewhere :) Cheers Phil