"Rough approximation" refers to an estimation or guess that may not be completely accurate or precise. It is a way of describing an answer or measurement that is close enough for practical purposes but not exact. Full definition
A dark day for rough approximations of food, as Hostess has filed a motion in bankruptcy court, asking to liquidate its assets and go quietly into that great night. (relevantmagazine.com)
Here's an interesting way to look at the problem from a California perspective, courtesy of my state senator, Joe Simitian, who offered these numbers as rough approximations in a community meeting on Saturday, February 5th, 2011. (teachhub.com)
To a very rough approximation then, the net carbon uptake by all of the planet's vegetation would need to triple (assuming similar transfers to stable C pools like soil organic matter) just to offset current carbon emissions every year. (realclimate.org)