It's really up to you, mixing is a good way to stretch your fuel system and don't worry about getting the mix exact, just run your car as close to empty as you can and fill with 20 or 30 litres of each fuel. Whether you have 2 or 3 litres in the tank before you mix won't be a problem. The problem is you will hate going back to 98, but if it's not available to you then you really have no choice. Race fuel is dearer but you will see more gains and if only using for drags is another option.