Interpolating between two graphs

Hi

I have scatter graphs created from reading taken from one off the books used to design crotch plates ( see attached). I have managed to do interpolation for various dt values at angles given by each graph. However I have a problem interpolating for an angle other than the one given a different degrees (eg. 53 degrees). The ultimate solution that I am looking for is to get dt value for any degree between 30 and 90 degrees.

Regards
Justmcd

You need to do the same thing with the angles as you have done with the db values.

I extended/repeated the last column in the lookup table to avoid any errors in formula.