Note that your qualification will depend on your 2020 tax filing; everything else is an upfront 'estimate'.
So if you get too little now, you can get it later through your 2020 tax filing (so in April 2021); on the other hand, the current policy seems to be if they gave you too much, you will not have to pay it back.
If your tax filing for 2019 would result in a higher payment than from the 2018 data, and you have not receive it yet, you should get the higher payment - if the calculation is not done before your 2019 taxes are accepted and saved on that server. There is no way to know for sure, but the longer it takes, the better your chance that they use the newer data.
Here is a source that explains it (https://www.irs.gov/newsroom/why-the-economic-impact-payment-amount-could-be-different-than-anticipated, last paragraph):
[...] EIPs [Economic Impact Payments] are technically an advance payment of a new temporary tax credit that eligible taxpayers can claim on their 2020 return. Everyone should keep for their records the letter they receive by mail within a few weeks after their payment is issued.
When taxpayers file their return next year, they can claim additional credits on their 2020 tax return if they are eligible for them. The IRS will provide further details on IRS.gov on the action they may need to take.
The EIP will not reduce a taxpayer's refund or increase the amount they owe when they file a tax return early next year. It is also not taxable and it should not be included in income on a 2020 return. [...]
