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 en.wikipedia.org/wiki/Calcium_…).

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.

chemical compound

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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.