6s vs. 4s?

[kingdomracer]

Right now I have Thunder Power 22c 8000mah 2x2s=4s w/1800kv MMM combo. I get close to 25-30 min. no prob but would like to run more efficient and save some weight w/5s, how do I get the same runtime from lower mah batts. Is there a formula that will calculate the needed size batts? Plus can someone help me understand how is it that with a lower mah which leads to a lower constant for most batts out there (usually right around a 100 amps constant) is a good thing? I've been lead to believe that if my MMM is rated at 120 then I should make sure my batts are at the lowest 150, I don't see that same rule applying in a 3300mah batt with a much lower constant amp rating. I thought when you buy a lipo it should be based on 80% of the motor rated amp draw. I hope I made sense and I will not take any offense If I'm confused in my thinking. Plus it would help for the guys who are running the 5s and 6s 1600kv-1800kv setups to post their runtimes. Thankz!