You know that 7*7 is 49, and 6*6 is 36. Now you know it will be 6._. 6*7 is 42, so think of that as 6.5^2. You know that it will most likely be less than 6.8 since it is so close to being 7*7, which is 4 above your desired number. This leaves 6.6 to 6.8 as your estimate. Personally, I would go straight in the middle and choose 6.7.
I hope this helps!