The easiest way might be to use a Python 3 Code Input. You can get the voltage measurement, perform the mathematical conversion, then save the new value with the following code in your Python 3 Code Input. Make sure to set the appropriate Measurement/Unit in the Python 3 Code Input options, as well as copying the correct IDs from the ADS1115 Input. You can find/copy the Input ID with the button next to the Input name, and the measurement ID under Configure Measurements next to the channel/measurement name. Enable Log Level: Debug to see the debug messages appear in the Daemon Log.
from mycodo.utils.influx import get_last_measurement
input_id = "REPLACE_WITH_ADS1115_INPUT_ID"
measurement_id = "REPLACE_WITH_ADS1115_MEASUREMENT_ID"
max_age = 1800 # Max age to query the measurement in the database (seconds)
voltage = get_last_measurement(input_id, measurement_id, max_age=max_age)
if voltage is not None:
water_level = (4.5*(1.56252*math.acos(-2.6)+(voltage-1.25)*math.sqrt(voltage*(2*1.25-voltage))))*1000
self.logger.exception("Error evaluating equation")