Hello mhartman

Welcome to Math Help Forum!

I'm not sure where the formula you quote comes from, but this may help.

The midpoint of the line joining the points and is

So in the example you quote, it will be .

The equation of the line joining these points is:

which can be re-arranged as:

which gives you the linear interpolation formula that will calculate the value of for any value of in the interval . So, for example, in the interval you quote, if you want the value of y when , it's

Grandad