"a safe guess" is correct and usable in written English.
You can use this phrase to mean an educated guess, which is based on your knowledge and evidence, and with a high likelihood of being correct. For example, "It looks like it might rain today - that's a safe guess since the sky is very cloudy."
Not to be like Finland is a safe guess.
Sitting next to Mr. Noy was a woman, age unknown, but a safe guess would be in early 20s.
It is a safe guess that nothing like an across‐the‐board equality of student‐faculty representation will emerge from these negotia tions.
It's a safe guess that few arbitrageurs or other Wall Street folk will be asked to point the way for the Class of 1990.
It's a safe guess that Aviana won't be watching her mother's take on Peggy any time soon, a role Ms. Adams admitted she didn't particularly warm to when she finally saw her on-screen.
It's a safe guess, anyway, that, while aware of a distant past containing such figures as postmen and delivery boys, we would be unable to imagine life without the pneumatic tube.
It's a pretty safe guess, then, that this is just hardball domestic politics -- though the peculiarly self-effacing behavior of Americans for Responsible Foreign Spending suggests that they fear that even today's hardened voters might look askance at politicians willing to undermine sensitive peace negotiations for the sake of partisan advantage.
When the FBI purchased plane tickets for the hackers, the originating flight was from Russia, so it was a pretty safe guess where the hackers were from.
This is a survival technique because if our brains perceive a sound that we cannot see then it makes the safe guess that it must be coming from behind us - in order for us to try and make sense of the sounds we are perceiving.
If I don't feel safe, guess where I won't be next Sunday," he says.
Mr. Cruz said his "safe guess" was that it happened about 10 times.
