It's generally hard for humans to estimate how far away an aircraft really is.
Question: Is there a standard rule for estimating the distance to an aircraft?
The entry/exit hatches are 2.4 metres high and your thumbnail at arms length translates to a 2.4 metre high door at 400 metres
Adults with normal 20/20 eyesight can make out an airliner's passenger windows in normal sunlight, so the plane is under 500 metres away, and over 500 metres if you can't make out individual windows.
Aircraft's registration code must be XX cm high and is therefore readable at up to 1000 metres.
Of course lighting and elevation differences, paint designs and model of plane, and even heading differences would mess it up a bit, but this is a rule of thumb in lieu of any other information.
For example, the popular fallout character is not giving a thumbs-up. Instead he's comparing the size of a distant mushroom cloud to his thumb to see if he's outside the nuclear fallout zone.