I did a quick search and came up with some interesting wording in the overview of an EPA report on greenhuse gases (which included CO2):
https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions
So, it appears the interest of the study is not what percentage is man made, but rather how much of the increase in the last 150 years is due to human activities!
Happily, I was mistaken in my earlier post. While direct measurement of CO2 levels is a recent ability, it has been established that reasonable estimates can be made from ice cores with the chart below showing data for a little over 2000 years published by the American Chemical Society.
https://www.acs.org/content/acs/en/climatescience/greenhousegases/industrialrevolution.html
Since red is CO2 and what appears to be the ambient, naturally occurring level of CO2 is about 280ppm and it is now at around 380ppm for an increase of 100ppm.
The earlier quote states that
almost all of the increase of green house gasses is from human activities.
"Almost all" is rather vague, I would normally expect at least 90%, but lets just
assume it is between 75% and 99% or between 75ppm and 99ppm.
That would mean the portion of the current level of CO2 due to human activity is 75/380 and 99/380 which converts to between 20% and 26%!
Please note the assumption I highlighted above. I don't know the real number. I believe my assumption is reasonable, but also believe more accurate info is available.
Also, I do not know that the relatively steady level of CO2 from the previous millennium is properly attributed exclusively to "rotting vegetation"!? So in that sense it may not properly answer your question.
HTH!