Super-technical threads need to respect significant figures. Your result can't be more precise than your inputs.

This is a sciency thing about precision. The classic example is the old joke of "this dinosaur skeleton is ten million years and two weeks old - they told me it was ten million years old when I started, and I've been working here two weeks", the point being that the age was roughly ten million years, not tell million years exact to the day.

Scientists talk about "significant figures", the number of digits that are actually important and not just extra zeroes, e.g. 10000 has one (probably!) and 0.0103 has 3. So when someone says it's 70°F they are probably rounding to two significant figures, so for the conversion you should do the same, i.e. 21°C. If they said it's 70.0°F then you'd convert to 21.1°C.

Sure, it's a simple unit conversion, but it's a rough figure. When I say it's 70 degrees, I'm not saying that it's 70.000 degrees.

What I'm really saying is that it's about 70 degrees, so the true temp could be between 69.6 and 70.4 degrees (I don't know, I'm not looking at the thermometer that closely).

69.60 F = 20.89 C 70.40 F = 21.33 C

Turning my "70F" into "exactly 21.11111C" is just silly.

Did no one else learn about significant figures in grade school?!