It's easy. Take whatever fuel you put into your tank in litres, add three zeros to turn it into mls. eg. 25lts =25,000ml. Now at 100:1 take off 2 zeros - 250mls of oil required. If you want 50:1, double it ie. 500ml.
So, to shorten this at 50:1, take the fuel you put into your tank in litres, double it and add one zero. eg. 15litres of fuel requires 300mls of oil. 12.5lts of fuel requires 250mls of oil. Gotit?
Use any TCW3 oil and it'll be fine. I've got a 100lts of quicksilver for sale very cheap.