How do you estimate sums and differences?

Once can roughly round to tens if the numbers are less than 1000, or to hundreds if the numbers are less than 10000, etc., trying to make up for large round-off errors which occur when the number are close to 1/2 the round-off amount.

E.G Add: 13+ 57+ 49 + 45 + 78 + 105

Add 10 + 60 +50 + 50 + 80 + 100