Good question! I don't know, but it should be possible to multiply the standard CO2 proportion of air by the density of air at your altitude by the volume of air in the greenhouse (216 cubic feet for a 6'x6'x6' greenhouse with nothing in it) to get the mass of CO2 inside at a given time. If you wanted to raise that by 10%, you'd just add that mass of dry ice - it has the same mass whether solid or gas.

But, air exchange with the outdoors will equalize that concentration gradient eventually, perhaps even as fast as the dry ice sublimates. Even in an airtight greenhouse, the enhanced CO2 levels will benefit some plant groups more than others, in a way that changes with day length and temperature. Timing probably matters too, with some plants shutting their stomata in the day.