Usually valuations are determined more by the investors rather than the founder. That might sound unfair, but investors have much better market comps. (Kind of like how you might think your house should be worth $X, but a real estate agent will tell you it's worth $Y, and their guess is usually better than yours.) If there are lots of investors vying to get into a round, then the founder has a lot more pricing power.

Market terms for a seed round are either 1) a priced round, or 2) a SAFE or note with a cap and a discount. 20% is a typical discount. A typical cap, in Silicon Valley, would be $4m-$5m on the lower end (idea or prototype phase, little or no market validation) up to $8m-$10m on the higher end (product is live in the market, has decent usage, and perhaps $25k-$75k MRR if you're b2b). There are also outliers that are >$10m cap -- usually those companies have exceptionally strong growth or founders.

In practice, SAFE/note discounts don't come into play that often but caps do. That is, next rounds are usually significantly higher than the cap (and the discount doesn't apply).