Water temperature is definitely the first parameter to pilot the final temperature of the Biga, which should be between 18°C (64°F) and 20°C (68°F). You can use this formula to get a reasonably realistic temperature: Water Temp = 55 - Flour Temp - Room Temp. !!! This formula only works with °C. You can apply the formula with °C and covert the result to °F !!!
You can also reduce the yeast percentage to 0.7% of the flour's weight. If the room temperature exceeds 30°C (86°F), add a small quantity of salt equal to 0.5% of the flour weight (5gr of salt for 1kg of flour).