Many power MOSFETs I have seen, fly source bond wires
into the core of the chip face. And they tend to use 10
mil wire, or today even ribbon bonding. This aspect is
very variable, by construction.
I'd say allotting 10-20% of total Ron to the wiring (chip,
bond, leadframe) is sensible enough. You might find more
insight by reading vendors' SPICE models, where they
will often break up the conduction path to insert Ls, Cs
and Rs realistically and leave clues as a result.