Control what you can as practically as you can. Holding water to +/- 1-degree C and holding times within 15 seconds... This isn't really hard.

I agree with Mark Barendt, and I often say that you can be off by 15% and still have a perfectly printable negative.

Controlling the variables tends to keep you within 5%, and when something goes awry... You still have a good chance of a successful shot. This means you approach 100% usable negatives in terms of technical qualities...

The real problem to control is making the exposure occur at the right time in front of the right subject...