Estimate the sum or difference subsequence

Note that Khan Academy has estimating sums for three-digit numbers in 3rd grade, which agrees with their decision to start rounding in 3rd grade. I decided to start rounding in 2nd grade, and so estimating the sum or difference can start in 2nd grade as well. I think each estimation topic should be taught around the same time as the exact method is taught, as estimation can be used to check whether the answer obtained by the exact method is reasonable.