you release energy by dissolving CaCl2 in water. The calculation for the amount of energy is given and explained here: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Introductory_Chemistry_(CK-12)/17%3A_Thermochemistry/17.13%3A_Heat_of_Solution. You can then calculate the temperature increase in your heatpack through the specific heat capacities of your compounds. That way, you can design your heatpack for a desired temperature gain.

What are you optimizing for?

IIUC adding too much CaCl2 will not hurt: it will just not dissolve once you go past the amount a saturated solution can take. So, if CaCl2 is cheap enough you can just add too much: the only downside is that your heatpack will have higher heat capacity (and so will use up more heat to heat itself up). Note that its solubility in water increases with temperate in a significant way (see e.g. the sidebar on https://en.wikipedia.org/wiki/Calcium_chloride).

I don't know exactly how quickly, but dissolution slows down when you get close to saturated solution. It implies that you might wish to be far from saturated solution so as to get a faster-acting heatpack. However, I expect that this effect can be ignored, because having more water means having more mass for the same total energy, so heating the heatpack up will become a larger factor in reducing the power provided to the outside.

tl;dr I would aim for around saturated solution at heatpack's intended operating temperature, unless you are optimizing for some specific unstated variables.

I expect that optimality would depend enough on what you care about: highest total heat delivered, highest rate of heat delivery over first XXminutes, highest heat delivery while keeping the heat delivery rate uniform, ...

Now that I think of it, if you want stable rate of heat delivery you might wish to feed water at a constant rate into a container with cacl2 instead of mixing it up at once.

I assume the situation you are considering is a person without a fixed structure around them, who's wearing all the wind/water protection and insulation they are using. I also assume you want something that will not require constant attention and so allow the person to sleep.

I'm curious what sort of container are you considering. (I initially thought that it's very important that it be well sealed, but I realized that as long as no solid calcium chloride leaks out, the only potential danger from the contents I can see it its high temperature.)