Ew sqrt(2.6) ?
In that case there's really two ways to think of it.
1.) Its gotta be bigger than one and less than two, by intuition. So a ballpark answer would be 1.5.
2.) You can get closer if you happen to know to sqrt(2) and sqrt(3), which are about 1.4 and 1.7 respectively. So a closer guess is probably 1.6.
This method works out more neatly is you have a larger number say, 53. The closest square roots are 49 and 64 which are 7^2 and 8^2 so you know that sqrt(53) is 7 and some change.
I tend to use your method, noospace, when confronted with multiplying decimals, like when I get an approximation from the above method.
Say I have 5*sqrt(53) ----> 5*(7.3) = 5*(7+0.3) = 35 + 1.5 = 36.5
Hope that's clear.