What is your grain temperature?

Here's the basic formula for calculating strike temperature.

strike_liquor_temperature = (.2 / hot_liquor_to_grist_ratio_in_quarts_per_pound) x (desired_strike_temperature - grist_temperature) + desired_strike_temperature

Example

grist_temperature = 25C/77F

strike_temperature = 66C/151F

hot_liquor_to_grist_ratio_in_quarts_per_pound = 1.5

strike_liquor_temperature = (0.2 / 1.5) * (151 - 77) + 151 = 0.13 * 74 + 151 = 161F (72C)

The trickiest part of calculating a strike liquor temperature value is determining the thermal loss to the tun itself. You will need to add 4 to 6 degrees Fahrenheit to strike_liquor_temperature to account for this heat loss. I would add 6 because you are using a large tun for the amount of grist that you are using. As an alternative, you can preheat your tun, and then just go with strike_liquor_temperature. If you do not mind overshooting a bit, I would use strike liquor that is 16 degrees (18 degrees will definitely give you room to adjust down) hotter than your desired rest temperature. As you have assumed, it is better to overshoot and cool than to undershoot and have to calculate the amount of hot liquor at 100C/212F necessary to hit your strike temperature.

strike_liquor_temperature_accounting_for_tun_loss (approximate) = (.2 / hot_liquor_to_grist_ratio_in_quarts_per_pound) x (desired_strike_temperature - grist_temperature) + desired_strike_temperature + 6 (Fahrenheit)

strike_liquor_temperature_accounting_for_tun_loss (approximate) = (.2 / hot_liquor_to_grist_ratio_in_quarts_per_pound) x (desired_strike_temperature - grist_temperature) + desired_strike_temperature + 3.3 (Celcius)