According with
Fenican's post, you can use this guideline to set optimal difficulty for your hash power:
Hash Rate (KH/s) Difficulty
1 +0.00000116
5 +0.00000582
10 +0.00001164
50 +0.00005821
100 +0.00011641
250 +0.00029103
500 +0.00058207
750 +0.00087310
1000 +0.00116414
1600 +0.001862624
Example: if you have ~50 KH/s, then add +0.00005821 to your address:
cgminer --scrypt -s 5 -o jkc.sytes.net:19327 -u YOUR_JKCCOIN_ADDRESS+0.00005821 -p x
After testing this configuration for 1 hour, the number of rejected has decreased.