acbus 1
Assuming they buy all compulsory DOs but you don't let them have any more, on average it works out as 10 off in 28 and a 5.3% payrise, treating that payment for DOs as part of the wage increase.
The other extremes are: 11 off in 28 and 1.5% (unlikely - we are going to be short of crews this summer),
...or 9 off in 28 (assuming you allow them to buy the others, which would be at the higher normal rate of £500 Capt & £300 FO) and about a 10% rise.
...and free car parking.