Agreed with @gandharba. You would put the exact amount from your joint tax return in i864 part 6-24. That amount is just for reference. What matters is 2018 annual income. Since your spouse has sufficient income, he/she will put estimated his/her annual income for 2018 in part 6-20 and provide evidences like current paystubs and/or employment letter.

Even if your spouse’s income was insufficient and your income was to be included, you wouldn’t need to fill in i864A as the spouse and intending immigrant but you would need to provide a letter from a employer that your employment will continue after you get your GC. No need to do that since your spouse’s income is sufficient.