With myvapemix if you only have percentages for flavours, you could create a recipe with a total of 100ml, then enter your flavours by ml where ml = %. Then you can "duplicate" the recipe to any other amount and the proportions will be kept.
Ie: 100ml with 13% flavour = 13ml. When you duplicate the recipe to 50ml the amount of flavour in the new recipe will be of 7.5ml.
This would solve your problem.
And again, yes you can enter all your flavours in a list including the amounts you have, the app will then calculate how much you have used and what you have left for each flavour. (by default the amount of drops (if you use drops) is of 25d/ml, but you can change the default value or even the value for each flavours if you want to)