Assume that, because you haven't seen the 'min insert' mark on the seat pin, there's loads of it left in the second hand bike you just bought.
Rode the
new-to-me Mercian to work for the first time in drop bar guise today. Got to work knowing that the saddle was too low, so raised it about 10mm.
On the ride home I thought it needed another 5mm or so and, because I had to stop to put the overshoes on anyway, eased the pin up but, before I could get to 5mm, out it came!
Previous owner had cut off some of the unused pin, including the 'min insert' marking. For half my ride home I'd had about 5mm of pin in the tube. That's a close enough call. I won't assume next time.