I don’t think you would want this.
While it’s certainly high, which is good, I don’t think that 85% is enough to change the default behaviour of your forum.
As far as I know, pretty much any (I’d bet on 99+%) https site will redirect all http requests to the https one.
The reverse does not hold true.
Meaning that linking to http://example.com is almost never a problem, since you end up on https://example.com anyway.
Like @Stephen , I think you might be focussing on the wrong part. As far as I know, having a <a href="http... link will not trigger the mixed content warnings.
This error should only be triggered by embedding non-https content into your https site. So loading external images could be a culprit, or embedding a third party script, something in a theme maybe?
Probably good to dive deeper into the browser console and see what triggers the error 