When I'm doing woodworking and I need to add up some very simple measurements I always use a calculator. I could very easily do it in my head and often automatically do without even thinking about it. However I've learned the hard way it's very easy to make a simple error with even something as simple as 8+7 and chances are you won't find out till later when two parts don't fit together. That's 20 hours down the toilet, just because I didn't double check using the shop calculator.