I am now, have been and will be wrong

Pre-money valuation = what your company is worth 1 moment before the investor give you the check.
Investment = Amount of money on the check
Post-Money Valuation = Pre money valuation + Investment

New investor ownership % = 1- old investor ownership %.

Understand how all these numbers relate.

If you're going to ask for $5 million, and are selling 20% of the company to the investor, you are stating that after the investment is complete, the investor will own 20% of the company for $5 mil. Or that the company is worth $25 million after the investment.
This is also saying that the company is worth $20 million BEFORE the investor puts any money into the company.

Now, if you're going to ask someone to invest under the above conditions, WHY are you worth $20 mil right now? If you can justify it, then it's not unreasonable. Otherwise, the final number is always up to negotiation.