Best range finder

Using Arduino Sensors
6

It looks to me like rounding error. Have you tried 6762 / 27 = 250 , than next value 6762 / 28 = 241, nothing in between - integer math. I'd suggest follow tip #2, averaging results over 16 gives 2 additional bits of resolution 10bit ADC becomes 12-bit.
Two options:

  1. sum up 16 variables but do NOT divide by 16, better scale up 6762 x 16 = 108 192
    val=(108192UL/( sum_i_16 - 9)) - 4;
    Serial.println(val);

You, probably will have to adjust 9 and 4 coefficient as well, do you math in Excell or LIbreOffice.

  1. sum up 16 results of analog reading, but keep results of division as "float"
     val=(6762.0/( average16_float-9))-4;
    Serial.println(val);