I want to modify a Madeira recipe to Choc Madeira - how do I do this?

The normal Madeira recipe to bake an 8"x 8" square:

310g butter
310 g caster sugar
6 eggs
230g plain flour
75g Selfraising flour
6tsp milk

How much plain or selfraising flour should i substitute with cocoa?

I want to bake a 15'' x 10'' 'stiletto' shaped cake for my friends daughter's 16th B'day, how do i calculate how much ingredients to use from the above? What multiple of the above quantity should I use for this cake?

Please HELP!! thanks for your prompt response.

I'm afraid I can't help. I always thought Madeira was wine. So having seen the name of the cake previously I always thought it was laced with it. However, after going to Wikipedia, I see the name originated from the Madeira wine that was served along side the cake. One learns something every day.

Anyway, I did find you some recipes of chocolate madeira cakes which may help you figure it out.

http://uktv.co.uk/food/recipe/aid/607037

http://www.food.com/recipe/chocolate-madeira-cake-with-chocolate-glaze-366779

http://ickystickypudding.blogspot.com/2008_08_01_archive.html#2903183979815739121

http://www.bianua.com/documents/shop/Bia_Nua_Chocolate_Madeira_Cake.pdf

substitute 25g plain flour with 25g unsweetened cocoa powder dissolved in 2-3 tb water. Mix and check dropping consistency ..

Thanks one & all for your response.
I decided to just make a choc cake instead & found that putting it in the fridge for a couple of hours, made it firm enough to cut into a novelty cake - just as easily as a madeira cake (which is best suited for novelty cakes due to its firmness & low crumb).

I always appreciate the spontaneity of responses and willingness to help by viewers - thanks again