This is it. I did the calculations on the econmy tab, and it turns out the real percentage is 49,5!The calculation for the trade income percentage which is shown in your right image is bugged and doesn't include all income sources. At least war reparations are missing and maybe more. You can check your total income in the economy tab. I think the economy tab shows the correct total income and the correct trade income and you can calculate the percentage there. And AFAIK the mission uses these values as well.
To achieve a higher trade income, you could try to get more trade power in important nodes or let other countries steer trade to you or transfer their trade power. Or could do a little nation ruining and do things to reduce your other income sources(e.g. unstate provinces which give a lot of income, but have little trade power)
Many thanks for this!
I'll try to bring my trade up slightly.