Have you considered the use of dispersants or coagulants before adding the flocculant and mixing prior to the settling test?  If you have a froth retention ring in your concentrate thickener you do not need to worry about the supernatant froth, but you must be able to obtain a clear solution above the mud-line and below the froth.   

There are better tests to determine the requirements of your specific application.  The dilution, mixing energy requirements, mixing time and several other parameters can be determined to design a feedwell specific to your situation. Mentioning a "centre ring" implies that an outdated open entry design is currently installed.  Such an installation is normally problematic even in forgiving applications.

Concentrate should settle relatively easily if you are using the right reagents.  Froth is certainly an issue with conc but it should not impact your settling tests, in terms of interface determination - again if you are using the right reagents. The interface should form very quickly.  There are a variety of test methods but the graduated cylinder settling tests should work for at least scoping level tests for reagents.  

As mentioned, froth will cloud the overflow if it is allowed to flow over the lip.  Retention rings are really a necessity for float conc, sprays work but not 100%.  If you have a very stable froth you may want to consider altering your frother to an alcohol type like MIBC or reducing the dosage.  Also keep in mind that thickeners need a diluted feed to work effectively so settling tests should be performed at 10-15% solids (I suspect your conc is not much higher than this anyway.)

If you dont see an interface quickly try a different reagent.

Regards