OK I think I have the logic next step...

454g of sucrose yields 46 gravity points in 3785ml of water

Therefore 1000g yields 101 gravity points in 3785 ml

Therefore 1000g yields 384 gravity points in 1000 ml

So, if a malt is quoted at 300, it means that 1KG will yield:

1 gravity point in 300 litres or alternatively 300 gravity points in 1 litre

Therefore the yield % of the malt is 300/384 = 78.2%

Is that right?