At one NHBC a guy did a test on FWH, 60min and mash hopping.
Sent it off to a lab. normalized to the 60min being 100%, FWH was 110%, Mash hopping was 20%.
He brought in the samples of FWH and 60 minute. There were differences to the flavors beyond bitterness. The FWH was smoother despite the 10% higher bittering.
I suggested that now knowing the relationship, drop FWH by 9% to get it even with the 60 minute and increase mash hop by 5x to match the 60 minute and retry the experiment. I don't think it he ever did anything.
IIRC it was cascade, but this could be different depending upon the hop acid composition I guess.
The problem with your idea is that you're trying to match measurable IBU, not perceived bitterness, which is what really matters.