I find cooling helps keep things accurate. I've got into the habit (in summer when temps are above 20° C) of mixing the three solutions up in glass yoghurt jars and then putting them into the fridge and they obviously cool right down overnight. Then I bring them out the next day and stand them on the table at room temp. I bought a very accurate digital thermometer and just monitor the temp of the developer as it goes up. A steel dev tank can also be got to the right temp by filling it with water from the fridge.