Why do some companies seem to be worth $20mm and some $4mm? Because investors were convinced that was what they were (or will be in the near future) worth. It is that simple. Therefore, it is best to let the market set your price and to find an investor to set the price or cap.
The objective is to find a valuation with which you are comfortable, that will allow you to raise the amount you need to achieve your goals with acceptable dilution, and that investors will find reasonable and attractive enough to write you a check.