Know your PPO2 at all times.
Before you check it you should know what it will be.
Check it regularly.
Think/Know what will happen to it if you descend, ascend, press O2 or Dil add, purge your mask etc.
Many much more wiser people than me will probably chip in but IMO if you can know and visualize your PPO2 then you should avoid alot of surprises.
Oh and don't be a cheap ar*e- throw your slime away when its spent, change your sensors and o-rings and spread the lube around plenty.
Just my opinion
