I wanted to know how Silver, Gold & Platium work.

i.e my membership year is from 1Aug - 31 Jul. And I am taking my family on a round world trip next year from Jul - mid Sept. This trip will be in 1st class. I have worked out that we will do 4 Qantas trips before 31July where we all get Silver status. My points will then go back to zero. By mid August my wife and daughter go to Gold, shortly after I go to Gold, then in Sept we all go to Platium. So how long to we stay on Silver , Gold and Platium levels for? Also as I got to Silver before re-set to I use the requalify levels to more up or only the re-qual for silver then 700 points for Gold, then 1400 for Platium?

Thanks
Route is nan-syd-sin-nrt-hkg-fra-mad-tlv-lhr-jfk-mia-dfw-las-lax-sfo-hnl-nan. with trips lhr-oslo return and syd-nan return in economy.