The first thing I want to say to you is chill out when cooking – it’s only a plate of food. When cooking savoury food, you have in many cases the ability to add, substitute or adapt what the recipe states. For example, if a recipe calls for 400g (14oz) of mince and your packet is 500g (1lb 2oz), use the lot. Sweet recipes and most things with a combination of butter, flour and sugar need to be followed precisely.
Butter is always unsalted; it is much better for cooking with. In years gone by, the salted butter was cheaper, but nowadays they are more or less the same price.