Your calculations are way off.
One cubic meter of wood harvests about 1 tonne of carbon dioxide.
An average tree is 0.6 cubic meters so that's 0.6 tonnes of carbon dioxide per tree.
To reduce atmospheric carbon dioxide from 400 ppm to 300 ppm, you need to remove 777 gigatonnes of carbon dioxide. Your calculation was that 192 gigatonnes would be enough but it's not. Perhaps you are confusing gigatonne of carbon dioxide with gigatonne of carbon? There's 3.66x difference between those.
That's 1300 billion trees required. Not 16 billion.
It would be about 168 trees per person.
Where would you find enough land area for all those trees?
Edit: on second thoughts, assuming 720 trees per hectare, converting 36% of all agricultural land area (which is 5 billion hectares) to forest would be enough to eventually (after 100 years) reduce carbon dioxide concentration from 400 ppm to 300 ppm. However, you'd have to kill 36% of world population first. Or maybe they die because of starvation after implementing the plan as there won't be enough food for them. Well, on a more positive side, if meat consumption would decrease drastically (for example by artificial lab-grown meat becoming less expensive), then maybe we could achieve 36% agricultural land area reduction.
...or maybe it's too optimistic to assume an average tree is 0.6 cubic meters as that would give 720 trees per hectare * 0.6 cubic meters per tree = 432 cubic meters per hectare which to me as a forest owner seems way too optimistic. You can achieve maybe half that by letting your forest grow very old. So maybe 720 trees per hectare is counting all trees, including very small ones, not just large trees. So better make 36% agricultural land area reduction into 72% agricultural land area reduction. I'm not sure we can realistically achieve that, unless each and every person on this planet stops 100% of meat consumption immediately. There won't be enough land.