I'd have to say your estimation is probably close if you're speaking about professing Christians in the United States.
Go to your favorite Christian statistic site and browse the polls,
Click Here, warning it can be a little sad.
You'll find stats like only 60% of Christians believe in the God of the Bible. And 80% of Christians believe in an everlasting reward (heaven) but of those same folks only 60% believe in an everlasting punishment (hell). "Heaven? Sounds good to me. Hell? Nah I don't believe in that."
Apparently 65% of Americans are Christian which gives us approximately 212m professing Christians in this country, and we know that aint right lol. I live in North Florida, which is considered the Bible belt, my county has approximately 100k people which means there should be approximately 65k people filling the pews each Sunday Lol. Some well respected theologians estimate that only about 5% of professing Christians are truly saved. Using the 5% estimation would amount to 5k believers in my town, sadly I don't think there's half that in all the churches combined on Sunday morning. I attend a small church with approximately 150 people and I would say the 5% estimate could be pretty close at 7.5 Christians in my local congregation lol. I'm not even sure my pastor is Christian or understands salvation if he is. He just did a series on Saturday sabbath and how we should be keeping the law, which I could overlook if every other sermon he preached wasn't unintentionally works based. It's rough out there folks, hope y'all have a good church in your area.