The SBA and the Treasury have release guidance (18 page document) on how the 2nd round of PPP should be calculated for the 25% reduction of revenue 2020 Q vs 2019 Q.
For the majority of the business, the main impact was experienced on the second quarter of 2020 due to the lockdowns and restrictions in different industries.
Our recommendation is to calculate the revenues as deposits in the business bank account (remove any loans or grants as those are not revenues from your normal operations). Once you add up the deposits for 2019 and 2020 in an Excel file or similar, you can easily compare the change in revenue from each quarter versus the previous year quarter.
If any of those quarters show a decrease of 25% or more (and you already got a 1st round of PPP), then, you qualify for this second round of PPP.