# Calc - Find delta between a serie of points and a point

Dear all, imagine you have a serie of points with a certain (fixed) coordinate between them. I then have an additional point which might end up sitting between 2 coordinates of the above mentioned serie. What is the best way to find the delta between the serie and the point?

Here's an example:

Serie:

x y

0 2

1 4

2 5

3 7

4 9

Point:

x y

2.5 8

In this case the result should be the delta between 8 and the interpolated value of 2.5 from the serie between 2 and 3 hence: 8-6 = 2

Of course, I don't know a priori where the point would end up being and I cannot simply use a linear interpolation of the whole serie. What I would need is a linear interpolation of the interval of the seri (0-1, 1-2 ...) and the the delta between the point and this linear interpolation.

Thanks

A formula doing this in one cell is possible (and you may already have an idea), but

complicatedand difficult to adapt/enhance if once needed..I would therefore want to know in what way the result shall be used. Depending on the answer a solution based on a reasonable number of helper cells, and using only rather short and well understandable formulas might be preferrabkle.

See attached example: enter code here

Thanks. This is for budgeting reasons. Imagine you have a budget for expenses of 10k in a year. You can assume that the 10k are evenly/linearly distributed through the year so that on Jul 1st (50% of the year) your expenses should have been 5k. If on that date you have spent 5.1k instead, then you know you are 0.1k above your budget. This is easy to do. Now, imagine instead that your budget is not evenly/linearly distributed but you know how much you will spend each mont. For instance, you can assume that on January you will spend 1k, on February 0.8k etc. My point is really to understand, for this situation, how far I am on each day of the year.