Help with adjusting amounts for winter

Hi all.

I make the tartine bread.  

My summer recipe is..........

All Purpose Organic Flour 800g

Fresh Ground Organic Whole Wheat 200g

Water 780g

Levain 200g

Pink Himalayin Salt 20g

I want to add more levain in the dough because the days/house is colder and it is taking longer to complete the bulk fermentation.

My Questions are;

1) How much flour do take out of my recipe (above) if I want to add 300g levain??

2) In the dead of winter I like to add 400g of levain but am noticing the combo cookers are a little to small for that amount of flour.  So how do I adjust my numbers?

Thanks for your help!