Instead of integers, use floating point numbers and the problem goes away.
Rather than something this:
voltage = ((sensorValue1 * xxxxx)/1023);
Use this (1023 is not the correct divisor):
sensorValue1 = sensorValue1/5.0; //take average of 5 measurements
voltage = (sensorValue1 * 5.0 * 4.26/1024.);
For best accuracy you need to measure the Arduino Vcc with a multimeter and substitute that value for 5.0. For example, when powered by USB, Vcc may be around 4.5 V.
You may also need to measure the resistance values to take into account their inaccuracy. The ratio to use is (R2+R1)/R1 instead of 4.26.
Or, proceed the way you have and calculate a "correction factor" from the measured voltage. If I understand this correctly:
The values I get on the analogRead is ~570 when voltage at divider is 11.77v
Then the voltage calculation is
sensorValue1 = sensorValue1/5.0; //take average of 5 measurements
voltage = (sensorValue1 * 11.77/570.);
Finally, fix a serious error in your code (you were initializing the wrong variable):
sensorValue1 = 0;