How do I measure the strength of it & dilute it ?
You need a hydrometer. This is a wee float, with a scale inside it. The more alcohol that is present, the lighter the density of the liquid, so the hydrometer sinks a bit lower. You then just read off the scale how much alcohol is present. You need a seperate hydrometer for measuring the density of the mash, as this is generally > 1.
