Full Wave Half Maximum function

Dear all,

I am currently trying to find the formula to help me compute the FWHM for the given xy data.

But,

I only managed to get the first half maximum using

=Index(1:2,1,Match(C6,2:2,1)),

I have been troubleshooting for few hours and has yet to find any solution to find the 2nd half.

May I ask for help in regards to this,
Many thanks.