What is the correct temperature for storing home-made frozen yogurt?

-18°C (-0.4°F) is not too low for storage (as this would be the normal temperature of a household freezer, where I store my ice cream and frozen yogurt. I definitely wouldn't set the temperature higher if I store also other stuff in the same freezer).

For serving you would probably want to let it sit out a little bit, so that it softens up a little so that you can scoop it (same as with ice cream).

If it is way too solid, you maybe let it thaw too much before storing.

• Then, you can fix the texture by re-blending it in a mixer (you can get even more ideas if you search for recipes on "how to make a frozen yogurt/ice cream without an ice cream maker" - often it involves freezing in an ice cube tray for easier blending).

• Or by thawing it again and using an ice-cream maker to get the desired texture.

I have read somewhere (maybe The Perfect Scoop? But I don't have the book here to check) that the best storage temperature for home-made ice creams and other frozen desserts is -6°C. And I have the same problem as you, with homemade desserts being generally too hard, even without the presence of noticeable crystals. They just don't have as much overrun and dry matter as the store-bought ones, which are formulated for colder storage.

As Martin says, keeping other stuff at -6°C will affect the storage time of other products stored in the same freezer. If you don't have a freezer with separate compartments, you will have to decide between keeping it at ice-cream-friendly temperature or using it mainly for long term storage.