Just to note that the 60%/67% factor applies only to planned operations. Once you are en route, changes to circumstances 9weather, aircraft failures, runway availability) may mean you don't have the margin available any more, which is acceptable - indeed, it's the whole point of the margin in planning, to account for these circumstances.
Its not a Transport Category aircraft requirement - there's nothing in Part 25 about a 60% factor. It's only alluded to in 25.1587(b)(7). The factor (and it's also not always 60%) is in for example 121.195(b). It's an operational requirement only.