Does anyone know how to estimate Qu of the filter?

I have read a paper titled "Narrow-Band Microwave Filter Design", which is written by Daniel G. Swanson(This paper can be download from the website of Microwave Magazine). I am really confused about the approach in that paper by which the unloaded quality factor(Qu) is obtained. To calculate Qu, equation 6 below is used in the paper.

As we can see in the above equation, insertion loss(IL) should be known before calculating Qu. However, as shown below, equation 1, which is used to estimate insertion loss, can not be obtained without knowing Qu!

I think it is self-contradictory. Can anyone explain this to me or tell me how to obtain Qu using other appoach please? Thanks!

Added after 24 minutes:

I am sorry the second picture is not clear. I upload a new one below.

Hi TUCHENG,

I am not a specialist for microwave systems, however, I think both of your formulas use different kind of losses:

* IL is the "insertion loss" which - more or less - is a parasitic loss due to mismatches and other unwanted effects.

* In contrary, the loss parameter in the other equation is the "midband loss" which, for my opinion, is a typical design parameter of the filter.

I am sure there are some forum members with experience in this field which either can confirm or correct the above statements.