The best warm-up strategy is to use the fibonacci suquence to determine the weights, so for set 1, you lift 1 pound. Set 2, you lift 1 pound again. Set 3, you lift 3 pounds, and so on. The progession looks like this:

1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

So, you're done warming up whenever you reach whatever fibonacci number is lower than your working weight. To calculate the number of reps, you figure out which fibonacci number you're going to stop at, and start at that prime number and work backwards through the prime numbers.

For example, suppose your work set of zercher squats is at 455, then your warm-up will look like this:

1x43

1x41

2x37

3x31

5x29

8x23

13x19

21x17

34x13

55x11

89x7

144x5

233x3

377x2

I realize that the first few sets seem like very high repetitions of very low weights, but these numbers are found in many facets of nature, and will prime your CNS for maximum output.

Now, if you're lifting in kg, you'll have to divide the whole thing by the golden ratio ~1.61803399

I hope this helps! Let me know how it works for you.