I'm not really sure that is the normal rate for your hardware. I think you should be getting more, even without overclocking. My Nvidia 1080 Ti can get something like 880 MH/s with overclocking using the same
ccminerAlexis78 miner for the Skein algo. Maybe you should check the ccminer version and/or try other mining software. The best tool for overclocking that I know of is
MSI Afterburner.