possible to substitute tartine starter for fresh cake yeast? any conversion ideas?

I've got a foccacia recipe that calls for fresh cake or compressed yeast which I cannot find. I have never worked with active dry yeast but have a very healthy Tartine starter. Is it possible to subsitute my starter for the fresh cake yeast?

The recipe calls for about 15.5 ouces of flour to .39 ouces of cake yeast.

Forgive me if this is a completely insane idea...I just would like to make good use of what I already have rather than try to buy a new product that is very difficult for me to find.

Sincere thanks for any advice.