@Johnny
My bad - I totally misread your OP. Apologies, good sir.
If I understand your take, the results are:
Pirate 5: 97
Pirate 4: 0
Pirate 3: 1
Pirate 2: 0
Pirate 1: 2
However, there's a problem. We assume that all 5 agents are "perfectly rational". Therefore all five agents would be able to independently work out the mathematical equilibrium you posit. Agents 1-4 would be anxious to avoid this scenario as the possibility of more gold for each is available. This would be perfectly rational.
All know this is the likely outcome - because all are brilliant game theorists. Therefore the perfectly rational approach would be for pirates 1 - 4 to feign irrationality and demand nothing less than a fair split (20 coins is significantly better than zero, one or two) at the cost of killing pirate 5 and obtaining a larger, equal, split between the remaining four pirates.
Pirates 1-4, being perfectly rational, realise that Pirate 5 will realise this and, rather than risk rejection, accept an equal cut or a bargained offer, depending on how highly rank is enforced.
So a "rational" agent at best can hope for zero, one or two coins, but an "irrational" agent can obtain 20 or even 25. - Some rationality!
The rational approach would be to forget the rational approach.