For ex. I can find the maximum range speed by changing the power from cruise setting by 100 RPM and see what drop in speed I'll get. For ex, cruise is 2300 RPM, I set 2200 rpm and get a 5 kts speed drop, then I go to 2100 and so on. At 1900 RPM I get the drop more than 5 kts (approx 10). So I go back to 2000 RPM and lean the mixture. This is the best range speed.
Why do you believe this is best range speed? You don't seem to have taken fuel consumption into account at all.
To determine these values experimentally, you really need an accurate fuel flow meter, like a
Shadin. Fortunately these tend to give a direct readout of both endurance (max endurance is just min fuel flow anyway) and ground miles per gallon. Note that the speed for best ground miles per gallon will vary slightly with wind, so you'll need to find a still air day or at least, for an approximation, turn crosswind.