Warm up sets how many.

I have always been abit confused about warm ups. on 5x5 i did 1st set with 50%1RM x5 2nd set 60%x5 then 3x5 with 75%. I have some disk probs in my lower back so i dont train 5x5 that often i stick more with 3x8-12 but am confused with warm ups for 3x8, say i was going to use 60%1RM for my working set, should i do 1 warm up with say 50% then 2 work sets with 60% or 2 warm ups with say 45% then 50% then the last heavy set with my 60%1RM. how do you guys do it thanks

for example if you going to squat 200kg for 5 reps.
 
I would do this.
1) 60kg * 10 reps
2) 100 * 8
3) 140 * 4
4) 170 * 2
5) 190 * 1
 
I do a few light sets to get the blood flowing. then do heavier sets with less reps to get my nervous system used to the weight.
 

Thanks for the replay mate am going give that a try start realy light an work up to a real heavy set
 

In theory, so long as you're warmed up across the whole body, even just mimicking the movement to open the synapses is enough.
 
I tend to walk to the gym and then jog up the steps (5 flights). After that each exercise is preceded by 'shadow lifting' focussing on tensing the target muscles or working with an empty bar.