I see you are a T1 and understand that actual carb content is important to you and the couple of observations I will make are intended to indicate why your question is so difficult to provide a definitive answer to.
As regulars on here have probably picked up I am a bit of a number nerd and when it comes to carb content of stuff I have found it is a minefield. I'm with
@Kaylz, looking things up in apps and the like when you are talking about generic things like oats or spuds has to have a big "ish" put on the end of the number you are given.
One of the reasons for the lack of precision is that carb content of anything is actually quite difficult to measure. Take the humble spud in all its varieties and styles of preparation. Because it is hard to measure the carb content, nobody as far as I can see has got round to measuring the carb content of all the different varieties, growing conditions and time after planting let alone the effect of all the different methods of preparing them. Look up carb content of spuds in a number of sources and you will get quite different answers. Same must apply to oats, rice, and cereals and anything else with a significant carb content. You know that these things are up in the high range, but figuring quite what allowance you need to make for any particular set of circumstances is not easy. I'm guessing it has to come down to individual experience and getting an understanding of what works for you.
Labels on prepared foods are a bit more reliable, at least somebody somewhere has thought about what has gone into the product and although many values quoted are guestimates it's likely that they have been made by people with the knowledge needed to get it reasonably right.
Don't know if that helps but I like the idea of making your own museli. At least when you get a recipe that suits you and you can allow for consistently, you can reproduce it with confidence. I make my own granola for breakfast and stick with the same source of oats, fruit and nuts because I know it works.
