For K1 they require 100% of the poverty level, but that does go up to 125% once you get here and apply for the adjustment of status after you get married.
The best evidence is 3rd-party evidence, such as W2s, pay stubs, tax transcripts, etc. Anything that is impossible to generate yourself and is provided by a reliable 3rd-party is going to be the best evidence. If you have a weaker case, it is not recommended to simply provide tax return copies, but actual transcripts from the IRS showing that you filed and how much taxes you paid. Anybody can fill out a tax return and not actually file it. So the transcripts are a more solid version of evidence than simple returns.
If you are self-employed, you may receive more scrutiny on your evidence of income. This is a case where you definitely want tax transcripts instead of returns. And you need to make sure you aren't writing off so much income that your taxable income doesn't meet the income requirement. A lot of self-employed people tend to have a lot of write-offs and their bottom-line income is surprisingly lower than they think. If this is the case, you may be looking at filing using a joint sponsor to beef up your income situation. However, keep in mind that some countries don't allow K1 sponsors to utilize a joint sponsor, such as Cambodia or the Philippines.
If you are using assets, you will need 3rd-party proof and verification of the value. This may be stock portfolios, or notarized appraisals or any other proof other than "your word for it". Without 3rd-party evidence, your numbers are almost worthless.
