Your employer contribution to the solo 401(k) for 2020 is based on your full annual net earnings, not just he earnings from the date in 2020 that you established the solo 401(k).
However, your maximum employer contribution is not 25% of net earnings. Net earnings are your net profit from self-employment minus the deductible portion of self-employment taxes and your maximum employer contribution is the lesser of 20% of net earnings or half of your net earnings that remain after subtracting any regular (not catch-up) employee contribution to the solo 401(k). This is calculated on TurboTax's Keogh, SEP and SIMPLE Contribution Worksheet, but note that this worksheet has not yet been updated to use the 2020 limits for regular and catch-up employee contributions.
