For Municipal and School Board the number is much higher.
Really embarrassed to say that I don’t recall voting in the School Board elections. Only voted in possibly half the Municipal elections. Why? To me at the time it did not make a difference. Will I in the future? Hell yes.

It’s not scientific and is only anecdotal but in the past I have asked many people whether they vote in elections. The answer all the time was “it doesn’t make a difference”. They basically told me that they wanted to be left alone free from bureaucrats and people telling them what to do.
Is this the same for all 30-40% who don’t vote? No idea but what if it is all 30-40%? Then these people are possibly Conservative in their views or libertarian. Definitely not Progressive Liberals or they would be out there voting for more Insane policies. We know they are not quiet.

Conclusion: Could it be that roughly 70% of potential voters hold Conservative views. So encourage that group to vote. We may be pleasantly surprised.