iip3 calculation

Dear all,
I know the expression of
P_iip3=(3P_in-P_im,in)/2;
it leads to
P_in=(2P_iip3+P_im,in)/3,
to get P_in,max, let P_im,in=F (noise floor).
P_in,max=(2P_iip3+F)/3.
There are two variables in equation, P_in,max and P_iip3.
In RF system design, which one should be calculated firstly?

My understanding is as following: usually we know what we
want to P_in,max, and it will make us to give a reasonable
P_iip3. I am not sure it's right. Does the standard will give
figures of P_in,max or P_iip3?
Thanks
fxxjssc

IIP3 = Pin + IS/2 where Pin is power input level in dBm for each carrier and IS is the 3rd order intermodulation suppression in dBc (measured at the output of your system).

This expression is valid only as long as you are well below the input compression point of your system - this means well below P1dB input.