Grids and drawing chalk

About a month ago I decided to do a copy of Baburen's "The Procuress". This is as far as I've gotten. It's a 40"x 42" canvas with a light gray priming. The original painting is in the Museum of Fine arts, Boston, but I've never actually seen it. It used to belong to Vermeer, and it's in the background, hanging on the wall, in 2 of his paintings. When I was learning to draw, this was one of my inspirations; I liked the large, simplified shapes - it seemed a good way to begin a drawing. I also got the idea of using white drawing chalk on a light gray canvas from him! It might be a little hard to see , but this is apparently how Vermeer started a painting.
I found blackboard chalk to be too coarse and too soft, so I started making my own.Here's how - put one teaspoon of rabbitskin glue size in a small cup and then add another teaspoon of water so you have a size that's half the strength of what you would use to size a canvas or make gesso. Slowly add some powdered chalk (whiting), stopping to mix every now and then, until you have a thick sticky paste. Add more chalk til you get a dry paste, then take it out of the cup and roll it between your hands. Roll it into a cigar shape then fold it back into a ball; repeat a few times until it's smooth like soft modeling clay. If it's sticky, add more chalk. If it's crumbly, add a drop or two of water. Then take small amounts and roll into sticks of chalk and leave to dry overnight.
If it's too soft, use a stronger glue size. If it's too hard, add more water when diluting the size.