A breadboard may not be the most reliable connection, but generally it does not add 300 ohms of resistance. In practice it should not add more than .01 ohm of resistance (my estimate only).
Use your volt meter, and read the voltage from your voltage divider (at the analog input pin). Then do the math. What do you get?
No, don't measure the voltage like that, voltmeters have input resistance! Measure the value of the resistors, find out what the true
voltage divide ratio is (remember its R1/(R1+R2), not R1/R2). Then measure the 5V rail of the Arduino. This gives the information
to calibrate the voltage measurement properly. Resistors come in various tolerance ranges from 10% to 0.01% accuracym if you are
measuring you need to know the tolerance to see what your accuracy is.
An analog input has essentially infinite input resistance, unlike a multimeter.