How about using some other part of your bike as a measure?
For example, if ride a hardtail, put dab of paint/nail varnish/tape/etc on the top tube so that you could remove the seatpost then measure it against the top tube, hold it with your thumb then insert to correct height.
Or, don’t want to mark your bike in any way?
Shove a bung up the inside of the seatpost with a length of knotted string/fishing line/whatever so that folded back on itself it gives you the height of the seat. Whip the seatpost out. Measure. Hold with thumb at max insertion point. Insert seatpost. Just let the string dangle.
Or, don’t want to mark your bike in any way?
Shave the inside of your thigh. Put the seat at the right height then put your heel on the pedal and mark the inside of your thigh with an indelible marker at the height of the top of the saddle. If you wanted to make it more permanent you could get your leg tatooed. This method would also be good as a conversation starter in the changing rooms.
I’ll get my coat!
🙂