Strike water always runs hotter than your target mash temperature, because the grain itself absorbs heat on contact and pulls the mixed temperature down. For 12 lb (5.44 kg) of grain starting at 68°F (20°C), mashed in at a 1.5 qt/lb ratio to hit a 152°F (66.7°C) mash, the strike water needs to be 72.9°C (163.2°F) — and you'll need 17.0 L of it at that ratio. Reproduce this exactly, or swap in your own grain weight, starting temperature and ratio.
Why the number is higher than the target
The published formula (John Palmer, How to Brew) is StrikeTempF = (0.2 / R) x (T2 - T1) + T2, where R is the water:grain ratio in quarts per pound, T1 is the grain's starting temperature and T2 is the target mash temperature — the constant 0.2 approximates malted grain's specific heat relative to water. Grain colder than the target pulls the mixed temperature down when it's added, so the water has to start proportionally hotter to compensate — the colder the grain and the thicker the mash (lower R), the bigger that gap gets.
The two inputs that move this number the most
Grain temperature. Grain straight from a cold garage at 8°C instead of a comfortable 20°C room needs strike water at roughly 74.5°C for the same 152°F target and ratio — about 1.6°C hotter, because there's more temperature gap for the same water to close. If your grain sat somewhere cold overnight, measure its actual temperature rather than assuming room temperature; see why did my mash come in too cool if this is exactly what happened to you.
Mash thickness (the ratio). A thicker mash at 1.25 qt/lb needs strike water at roughly 74.1°C for the same grain and target — hotter, because there's less water relative to grain to absorb and hold heat, so it has to start with more heat in reserve. A thinner mash at 2.0 qt/lb needs only about 71.3°C. See what is a good mash thickness for how that ratio choice affects more than just this number.
Using this for a different grain weight or target
The strike temperature itself barely moves with grain weight — a 10 lb grain bill at the same ratio, temperature and target needs the same 72.9°C water, just less of it (14.2 L instead of 17.0 L), because temperature is an intensive property and volume scales separately. See how much water for a 10-pound grain bill for that calculation worked through fully, including sparge water on top of the strike volume.
If you're mashing in a cooler rather than a kettle with active temperature control, the cooler itself absorbs some of that heat on first contact too — see how to hit mash temperature in a cooler for the preheat step that compensates for it.