You cant overclock an i7 8700, certainly not in afterbuner that is only for GPU overclocking. Only the intel k chips can be easily overcloked, like the i7 8700k, and that requires a Z370 motherboard.
There is no specific number I can give you for afterburner, all GPU's overclock a bit different. I would start with a core clock offset of 50mhz run a benchmark, then go up 20mhz run a benchmark and repeat. When the GPU is no longer stable back off to a point that was and that is your max core overclock.