Cornstarch as a thickener — what ratio?

How much cornstarch should I use to thicken something, as a percentage of the weight of the food to be thickened?

I've seen guidance like "For thickening with xanthan gum, use 0.02 to 1.0% of the weight of your sauce, depending on your desired thickness. Much above this range, it starts to resemble mucus, and much below this range, its thickening is barely perceptible". Is there similar advice for cornstarch?

This sort of advice usually ignores any specific details about whether you make it into a slurry with water first, whether you use a blender while mixing it in, or how you gelatinize it.

Typically, the ratios are given based on the amount of liquid. The following is listed both on the packaging in my pantry and in my “basic cookbook”.

• For soups and sauces 10-15g per 500ml liquid (1/2-3/4 tbsp per cup)
• For puddings 40g per 500ml, up to 50 g for extra firm puddings (2-2.5 tbsp per cup).

Now, note that this is easy if you start from scratch, with known quantities of liquid. Particularly when making sauces there are typically two issues,
a) you don’t want to start measuring the remaining liquid after possibly a few steps and some reduction and
b) you may have already some texture/thickness from ingredients like a mirepoix.

In those cases, eyeball the amounts, make a slurry with cold liquid as usual, but add it only gradually to the dish, giving it time after each addition to cook and thicken, then add some more if needed.