I have a lookup table.
$X$ values are:
$\{230, 220, 210, 200, 190, 180, 170, 160, 150, 140, 130, 120, 110, 100, 90, 80\}$
$Y$ values are:
$\{2590, 2590, 2590, 2580, 2540, 2460, 2260, 1870, 1240, 1040, 960, 800, 680, 480, 360, 160\}$
I tried to create a formula using least square approximation. However, the accuracy is quite bad. Also I tried to split the data into $3$ different datas where the curve looks like linear and tried the same method. I could not get good solution.
I did not study math but I try to learn math methods to solve my problem. How can I solve this issue?