Two possible ways:
1. if you have the formula for the line, use it go calculate the reading.
2. If you have sample points on the line, but no formula, then you can use
the FORECAST() function. If you have two points [x1,y1] and [x2,y2] and you
also have a third x3, then FORECAST() will calculate y3 for you. The neat
thing about FORECAST() is that it works equally well if x3 is between x1 and
x2 or is outside of the range.
