Hmmm,
I usually just guess at how much cabosil I will need for the job, pour it into a mixing cup, then add mixed resin until it is about the consistency of paste (the stuff pre-school kids use for art projects). I think the mixing cups I use are 3 ounces, and I usually fill them about 1/3 to 1/2 with cabosil then add resin.

You only want to use enough to make a small fillet in the inside corners where the cloth will not make contact with the mold. If you use too much it will only add weight/thickness, and weaken the part. If you use too little you will likely get air pockets.
Should be just enough to form a little "ramp" for the cloth to follow up the corner.