Sorry, but this is how I was taught and how I've seen a lot of VC's do it. They take their assumptions which include revenue projections, required rate of return, risk premium, future dilution, and of course the tax rate. They then plug those into a model to tell them what percentage / valuation they need. Of course this doesn't work well for seed stage since there's not enough information.

Don't be sorry, but think about it. Your goal as a VC is always to maximize profit. If the US capital gains tax rate goes up, that doesn't change your outlook on US based investment on a per-case basis, it just makes all of US investment look "worse" compared to other countries with lower tax rates.

And honestly, that would probably not even be that bad. One of the major reasons techies talk so much about immigration reform is that people with ideas want to go to the US because that is where all the money is because nobody wants to invest outside the borders due to a combination of many factors, but tax rate is always one of them. A higher capital gains tax rate makes other markets more appealing, and I don't have the exact numbers on what other markets would be most appealing at what % hikes, but I can't imagine taking the tax rate up to even 40% would drive investors out of US markets just on the basis of how convenient America is for tech entrepreneurialism already, it reduces a ton of the risk factors involved you described.