To be honest what I do is I put all chemical bottles into a bucket, fill the bucket with hot water and start monitoring the developer's temperature and when that gets to about 38.3-5 degrees Celsius I start the process. After 3:15 in the tank the developer has cooled down to approx 37.2-5 anyway so its better to start at bit over 38 degrees anyway. I find it difficult to measure the exact temperature due to having to always shake the developer bottle thoroughly before taking the reading, as the warmer developer rises to the top of the bottle. For this reason I'm not 100% certain of my readings but I've gotten good enough results (my only problem is a purple cast on the negs, which is easy to remove in LR but annoying nevertheless - at the moment I'm guessing that this is either due to insufficient washing during washing cycles, or due to the local tap water).