Take a piece of veg tanned leather, dip it in some water for a moment, then lay it out to dry. Every 10 minutes, stamp it with a good sized stamp that has some detail in it. Mark the time. Watch to see the difference as it starts to dry. When it is close to returning to its normal colour, switch to 5 minute intervals. Continue until the leather is dry. This will show you what too wet and too dry does to how the impressions look after the leather has completely dried. If you use the same thickness of leather, and treat is the same way, you can literally time at what point to start tooling, and the point at which you need to add a small amount of moisture to keep the leather at the condition you want while tooling.
Tom