Buttermilk mixed With regular milk?

its probably pretty  much the same. Most recipes that call for buttermilk or sour milk are looking for the natural acid in those milks. Thats why adding lemon juice, a natural acid, mixed with milk makes an acceptible substitute. 

When I've been baking and used up all the buttermilk, I've just rinsed out the buttermilk carton with a little milk to get the last of it from the carton, then I add it to my baking mix or use the 'mix' to glaze my scones / biscuits.

I haven't used a larger amount of the blended mixture though.

Firstly be sure that you're buttermilk is live.  If so I prefer to add buttermilk to milk, or the other way round and let the jug sit on the counter under a clean kitchen towel so it can breathe.  It takes between 12 - 48 hours for the milk to thicken up and sour depending on the ambient temperature.  Now you have more buttermilk.

If you just need some for baking adding lemon juice or vinegar to milk to sour it will work out okay but you don't get true buttermilk.  It doesn't keep and is no good for drinking or pouring over cereal.

Buttermilk is easily made with whole milk and white wine vinegar. 

You can do that for a recipe if you need a bit more Buttermilk.  I would not do that for long term storage of buttermilk.  Like adding to it like you would a sourdough starter. 

Add the milk that you need to the buttermilk you have and let it sit at room temperature for 15 minutes or so before you use it.