TBH every tyre I've had doesn't feel right on overbanding, white lines etc no matter how much tread there is on it.
I had a similar question a few months back Flashman see
THIS THREAD. Some useful responses to it. I know you can get proper tyre gauges, but as I already had a Halfords micrometer (for measuring precise bolt widths etc) and it does the job brilliantly. I was bored one day, so took 10 readings on both front and back tyres, then averaged them out.
Make a note of the mileage and tread depth (in my case it was 10,500 miles, and 3.6mm tread IIRC) and repeat the process again a few months later to see how much tread you lose over over a given distance. It may be useful to work it out before a long trip in deciding whether tyres should be changed before going, or whether you've got enough tread to do the trip and change them after.