Yep, it really is a complicated balance of a range of factors (e.g. commercial income, wages, amortised player values, payment structure for incomings etc.)
Even the finance guys who do a deep dive on these things can, in lots of cases, only offer an informed best guess on whether clubs have overstepped the mark.
And can drive some tough decisions - often the guys you want to offload aren't the guys who will help your FFP position. And the guys you don't want to lose can really help your FFP position.
If Arsenal offloaded Saka, for example, they could spend like drunken sailors