Interpolation Calculation

I am trying to write a calculation that will determine 300 CSF. It is an interpolation calc.

(((Desired freeness of 300-lower freeness value)/(higher freeness value - lower freeness value)) *(revs or strength value corresponding to higher freeness value - revs or strength value corresponding to lower freeness value))+revs or strength value corresponding to lower freeness.

Example: Strength value - stretch

The 299 is the first value lower than 300 when increasing revolutions (0, 1000, 3000, 5000).

((300-299)/(369-299)*(2.35-2.50))+2.50 = 2.498

300 = desired CSF
299 = lst CSF lower than 300
369 = previous CSF or CSF to the left of the lower than 300 CSF
2.35 = corresponding stretch (desired property) value to the 299
2.50 = corresponding stretch value to the 581

The tricky part is that I need to say that if the 424 cell does not meet the criteria of the <300 then I need to check the 369 cell. If that one works, then do the interpolation using the 424 and 369 values for CSF and the 1.97 and 2.35 values for stretch. If the 369 cell is not < 300 then I need to check the 299 cell. If the 299 cell is <300, then I do the interpolation using the 369 and 299 values for CSF and the 2.35 and 2.50 values for stretch.

I do not know how to use tools, record new macro feature to get in all the ifs or to put in the 300.

Revs 0 1000 3000 5000
CSF 581 424 369 299
Stretch 0.87 1.97 2.35 2.50