Phil that depends on your mix. I'd assume you are on 50:1 but if the engine is really old it could be 25:1 and a few run on 100:1.
If you put 10 litres of fuel in, you add 10000ml divided by 50 for 50:1. so 1000/5 or 200ml.
If you put 13 litres in it'll be 260ml, or 14 litres 280ml of oil. BUT if its really a 14litre tank is there room for 280ml extra... It may be easier to work backwards - use 250ml of oil which will be easy to find a suitable size bottle, and put 12.5litres in from the pump.
What I find much harder is when I have a ""half"" full tank already. You have no idea how much is in there. Lets say its got 7.7litres in it and you want to top it off to 14litres. You go to the pump and add 6.3 litres. You now get the calculator out and divide 6300 by 50 and now need to find something that will measure 126ml...? Often easier to work with a 5litre can like you'd use for your car. 5litres + 100ml oil. Usually enough space to squeeze it in. Mix it, slosh it into the tank. If no room you just store the remainder in the can. Next time you fill up slosh the remainder in the tank first so you have an empty can to begin with...