That's a good general rule of thumb, but it's not always perfectly accurate.

Here's why:

* The geothermal gradient varies: The rate at which temperature increases with depth is not consistent everywhere. It can be influenced by:

* Location: Geothermal gradients are higher in areas of volcanic activity and lower in stable continental regions.

* Rock type: Different types of rocks have different thermal conductivities.

* Water content: Water can transfer heat more efficiently, leading to higher gradients.

* Average is not exact: The 25°C per kilometer is an average value, not a strict rule. You might find regions where it's closer to 20°C per kilometer or even 30°C per kilometer.

So, while it's a useful approximation, remember that the actual geothermal gradient can vary significantly depending on various factors.