Going by Hamelman's own instructions for a liquid levain (p. 358), the starter he is looking to use in most recipes is 125% hydration. In his formula, he feeds 5.5oz or 156g of old starter with 2.4oz or 68g of flour and 3 oz or 85g of water for a total of 10.9 oz or 309g of Chef stater. That's about 6 times more than I keep. But then I don't make 22 loaves of sourdough bread every day.

Myself, I keep my starter at 100% hydration as it's a lot easier to account for the water and flour you're adding should you need to adjust anything. If you want to make a regular yeast recipe but use starter instead, you can add ANY amount of starter and know that half that weight was water so it's simple to adjust the recipe.

Since all the recipes in the book that use starter (that I've checked out so far) will give you the steps required to build a levain beforehand. These don't typically use more than 2 or 3 tablespoons of starter (1 to 1.5 oz or 28 - 42 grams) for the Home versions so even if your Chef starter is 100% hydration instead of 125%, that small amount at 100% won't really matter too much. We're talking a difference of 3 or 4 grams of water in a recipe that usually yields over 3 pounds or 1500g.

However if you wanted to "stick by the book", then you would want your Chef starter to be 125%. Hamelman has instructions on converting your liquid starter to a stiff one for those occasions you may need one. See page 362.

How "thick" is 125% starter? It's like pancake batter. 100% is like stiff pancake batter. At least when you first mix it. After the yeasties have had their fun, the consistency liquifies a little.