Elderflower champagne - No fizz, how much yeast solution to

[jack_c]

Made a batch of Elderflower Champagne. Left it to ferment dry, primed and bottled.

It's flat.

Checked gravity and the priming sugar hasn't been touched. Guessing I left it too long before bottling.

I've used Champagne yeast and made a starter of nutrients, dextrose and let it ferment. After a few days I started adding equal parts of the Elderflower champagne to acclimatise the yeast to high alcohol levels. I now have an active 2l of starter that's alive, healthy and of equal or greater abv to what's in the bottles. Great!

So, how much starter should I add to each 750ml champagne bottle to be sure it will consume the dextrose in the bottle's and give me some fizz? I'm plucking the figure 20ml out my my hat but it's just a guess.

Any suggestions?