GAMTClub tests have some great questions for practice and the question pool is large enough for enough practice sessions.
Also, to built my stamina and get myself more familiar with exam timing constraints, I have take multiple gmat club quant tests.
I am not sure of what kind of algorithm they use to predict the quant percentile but I have enough confidence in Bunnel/BB that they might have done all sort of research (great example is their post on importance of order of questions in GMAT) before finalizing on the algorithm. So, In addition to great practice material, it also provides with you rough idea and where you stand.
Also, there are some great analytical tools to help you with post exam mistake analysis.
All in all, I feel, gmat club tests are must have for anyone preparing for gmat.