Using IF-based array formula within INDEX(LINEST()) formula

Hey folks,

Trying to get this formula to work. I have it working on my sheet when I use SLOPE() and INTERCEPT(), but since I am calculating the equation for a 3rd order polynomial, those won't do.

Here is my formula:
=INDEX(LINEST(IF('Backend Analysis'!$AC$3:$AC$367<200,'Backend Analysis'!$AC$3:$AC$367),IF('Backend Analysis'!$AC$3:$AC$367<200,'Backend Analysis'!$D$3:$D$367)^{1,2,3}),1)

The problem is that I am getting the same answer as if I enter it without the IF-based array filter, which is this equation:
=INDEX(LINEST('Backend Analysis'!$AC$3:$AC$367,'Backend Analysis'!$D$3:$D$367^{1,2,3}),1)

The spirit of the formula is to only use values from the AC array and the D array where the value in that same row of the AC array is < 200 (200 is a stand in, this will ultimately be a dynamic formula, but if I can get it working with a hardcoded number, it's easy enough to do the rest)

I suspect it has something to do with the fact that linest, itself, is an array formula -- just not sure though. Any help would be much appreciated!!

T