Sorry, only just found this. I feel I've let the speed of the prompt answers to strange maths questions side down .
In the image I used the example with £140 to work out the relative ratios you wanted (i.e. the discount curve) based on the ratios given. The £140 is irrelevant so long as you're happy with the respective discounts (in fraction/percentage terms) for each jump in quantity. If the example had been based on an item selling for £345.32 for one item it would still work out the same assuming you were happy with the 0.45, 0.28, 0.23, ...
As mentioned above, the other way of doing it is by starting with the 300 @ £x and multiplying x by 1/(whatever your max discount is). For the case of 0.2 as your max discount that would be 1/0.2 = 5. From that point you've got what you started with and you can then make whatever discount ratio/curve you want.
I was lost by stirlingpowers reply too!
Edit: For any discount amounts the above way is probably the way to go and use something like the following:
So all you need is how much you want for 300+ (or whatever) items and a discount 'curve' for the various quantities (including the maximum allowable discount for the 300+) and you'll be able to work out the respective values from there.