It's so easy to get things to 68°F it's not even funny. If you are souping when the chemicals are below 68, you set them in a warm water bath. If they're too warm, you set them in a cool water bath. You can make warm water by turning on the warm water tap on your faucet. If you need to cool things down, you put a jug or two of water into the refrigerator, or use some ice from the freezer. Predictable results require consistent procedures. Shoot identical film frames shot under identical conditions using identical settings. Developing one at 64° for 13:00 will result in a different image than developing the other at 75° for 9:00. This is due to the logarithmic (non-linear) nature of developing. Highlights do not develop at the same rate as shadows. This has been known for over 150 years.