What kind of sponge? Victoria sandwich type ( ie cream fat and sugar together, add eggs gradually, add flour) Or Genoise type - whisk eggs and sugar together over heat and fold in flour (fold in melted butter at end if you want) ?
Mary berry says 8 oz and 4 eggs in 2 tins to make a 8 inch cake. I think that's a big cake! I use half that to make a 7 inch in one tin. If you are just doing one tin I would go for 8 maybe 10 oz? Not that helpful!!
Basic quantities for a victoria sandwich 1 egg to 2oz sugar, 2oz butter and 2oz (SR) flour For a 10 inch, (reasonably thick) I would use 12oz flour -but if your are replacing 6oz of that flour with 1 oz of cocoa I'm wondering if you don't normally use more flour -make a larger quantity -I'd be worried that might curdle (not enough flour/cocoa)? (actually recently I've been using the weighing the eggs method - weigh eggs in shells and use the same quantity of butter, sugar and flour -so for 12oz flour - you weigh 6 eggs in shells -but use the same amount of flour etc as that weight and I've been really really impressed - guess allows for differences in egg sizes...)