Price calculation

Thanks for this plugin. It looks like it will do exactly what I’m looking for (I want to add an extra variation to all my product without having to do it manually). However, all of my products are already variable and hence do not have a base price (each variation has its own). How will your plugin calculate the cost? I see you can insert a % plus or minus, but what starting price will it calculate from? I didn’t want to just try it since (if I understand correctly) it will permanently add a variation to all products – which wouldn’t be easily reversed. Thanks for your help.