How Much Should Botox Be Diluted?

The dilution is not what matters, but how many units are actually injected. You can get a good result with each of the dilutions you mentioned, it really depends on the doctors experience and preference.

Dear Mutti
There is no single right dilution. Doctors will typically pick a preferred dilution of the BOTOX and work with that dilution. The vial comes with 100 units of Botox. So diluting with 1 ml produces a solution with 100 units per milliliters.
Diluting with 4 ml produced a solution that is 25 units per milliliters. Some doctor boast about diluting with 20 ml which means that each 1 ml syringe contains 5 units of BOTOX. This is too dilute.
Beware that there is an additional treatment called mesobotox make from 5 units of BOTOX in 10 ml of salne. This is so dilute that is not useful. More important is to go to an office and a doctor that you can trust. So that when you pay for a certain amount of BOTOX you have confidence that you received this treatment.
Dr. Steinsapir

This really varies based on individual experience.
The vast majority of practitioners follow the Botox Cosmetic package insert which suggests a dilution of 2.5.
However, many outstanding physicians feel they have better control with a dilution of 1 cc.

The dilution does not matter since different doctors prefer different dilutions for injections in different areas.  What matters is the number of units being injected.  If the solution is too dilute and not enough units are used, you will not have an effective treatment.