Converting malt yields from L/Kg to SG

I'm sure this has been answered before but couldn't find exactly what I wanted in the forum history.

My maltsters quote their extract figures as follows e.g.

Maris Otter
Extract fine (as is) 296
Extract coarse (dry basis) 303
Extract coarse (as is) 293

Now, how do I convert this to 'Potential SG'?
From what I understand of my existing figures I would use the extract coarse (as is) figure and that means that 1kg of maris otter will yield 293 litres of wort at 1.001 but I can't make the next step!

You can start with How to Brew page 190. 

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?

I'll have to refer to my copy of the book.  I read it when I replied but did not commit to memory.  It sure sounds right though.  I recall researching the differences between fine and course and seem to recall not using the fine values as they refer more to the congress mash.    There were a couple excellent Brewing Network episodes about maltster reports.  I thin a person from Briess was on an was really great!

I can't fault my own logic  - lol
But I'm puzzled that the value I get out is under 80% which is significantly lower than the 85% which is the guideline value in BeerSmith.
That'll make a heck of a difference to the grain bill!