hello, happy you were offered this work. i agree with what has been said.
my 0.2 (i ve been chef/manager in various producing plants and directed the brussels comics cafe opening):
regarding food cost and waste control, its just going further the habits you must have develloped as a sous chef/cook (what is a line cook btw? chef de partie? commis?).
regarding the dish creation, nothing very difficult, if you can keep the ratios and have the house philosophy. i dont know what will be asked to you but keep in mind there is nothing more complicated that to do something simple. Be rigid toward yourself, softer to others. always try to be an example. bear in mind you still have to learn in every domain (like everyone else)
the hard part is management imo. you'll need to be respected and a good sous chef to rely on. he will save you huge amounts of time and money. be prepared to fire people. you ll have to at some point and the first time is VERY hard for a normal minded person. try keeping an even mood whatever happens, and always use the hierarchy way to say things (ie if you talk to a low end cook about his mess, you kill your sous chef's authorithy in the kitchen, making impossible for him to help you effectively). this is very important because days only last 24h
best of luck, well no this is not about luck. best of courage!