"based mostly on" is correct and usable in written English.
You can use it to describe something that is based primarily on something else. For example: My paper was based mostly on research from the library.
Analysis was based mostly on 3 strains, one from each race (clade).
The awards will now be based mostly on test results.
To communicate they developed a creole based mostly on English.
But their case was based mostly on circumstantial evidence.
The diagnosis is based mostly on history and physical findings.
Still, the overall record of decision-making approaches that are based mostly on intuition is far weaker than the record of decisions based mostly on data.
The audit, though, was based mostly on accident reports from the railroads themselves.
But that trip may be based mostly on wishful thinking, foreign policy experts said.
As the process works now, consumers choose mortgages based mostly on the interest rates.
Ms. Sinzdak said the biannual report was based mostly on statistics from 2002.
Conclusion: probably a bit, based mostly on how much time he spends on the ice.
