Trying to write a IFS function looking at columns with IF formulas in them already

SGeorgie 

Perhaps you could give bit more details.

Correct Fee formula looks like

=IFS(
    AND([@Sample] = 1, [@Block] <= 499),
    "45",
    AND([@Sample] = 1, [@Block] <= 999),
    "52",
    AND([@Sample] = 1, [@Block] <= 1749),
    "66",
    AND([@Sample] = 1, [@Block] >= 1750),
    "80",
    AND([@Pets] < 1, [@Block] <= 499),
    "35",
    AND([@Sample] < 1, [@Block] <= 999),
    "42",
    AND([@Sample] < 1, [@Block] <= 1749),
    "56",
    AND([@Sample] < 1, [@Block] >= 1750),
    "70"
)

Skipping that it could be simplifies, why [@Pets] inside instead of [@Sample] ? Plus, most probably, you shall use numbers instead of texts. For example, 70, not "70".

Next formula,  if i over type the formula in P4 and/or 'Correct Fee - there is this formula, in P4 or within the Table?

Assuming within the Table it probably shall look like

=IF( OR($P$4="", $P$4 < [@[Corrected Fee]]), "Too Low", "Ok")

ISBLANK() doesn't work if you return empty string for now result.

Anyway, it's not clear what combine with what.