I read this Keeping time without an RTC - Microcontrollers - Arduino Forum. But I want to do a precise timing on atmega328p on internal 8 mhz clock. What values have I use?

The internal oscillator is only guaranteed to be within +/-10% (at room temp and 3.3~5v, it's usually within a few percent), and it varies with temperature and voltage. It's less accurate than the resonator used on most Arduino boards, which is in turn less accurate than a crystal (which is less accurate than the crystals used in an a typical RTC). So if you're hoping to do "precise" timing with it, you're going to be disappointed....

I just want to try It. What value have I use?

If talking about the numbers jremminton posted there - half the clock speed, means half as many "clicks" to use that terminology per second - so divide that 15625 by 2 and substitute into the calculations below.

I think that's a really dubious approach though - it's not clear to me why using millis() for timing is any worse....