Now if you want to take advantage of the full 7,500 credit you can increase your taxable income (like convert a traditional IRA to a ROTH IRA or sell some stocks with a gain). That will increase the tax on line 16 but even though it won't increase your refund you won't pay tax on the extra income. Then you won't be paying tax on that income in the future. If this confuses you just ignore it.
Basically you can only reduce the TAX on line 16 & 18 to ZERO. Then you just get back all your withholding, not any more.
