We know there have a relation between water/cement ratio and compressive strength of concrete. This relation is also valid for lightweight aggregate concrete in the identical way as that for normal aggregate concrete.
In many cases, usual procedure used for normal mix selection can be followed for lightweight aggregate concrete. But it is very difficult to determine free water remains in mix to take part in the chemical reaction.
We know mixed water, even in normal aggregate concrete, may be absorbed by the aggregate and after absorbing remaining water take space in the concrete. The remaining water becomes a part of cement paste.
Same condition in valid for lightweight aggregate concrete too, but the absorption is too much and it is very difficult to predict how much water is absorbed. The absorptions of different lightweight aggregates are also difficult.
Now problem is not mainly with the high absorption of water, the problem is to predict rate of absorption of lightweight aggregate. It is found a wide range of rate of absorption of different aggregates.
Some aggregate shows significant rate of absorption for several days after mixing or placing. Again determination of specific gravity in different state of moisture (say saturated and surface dry) is very difficult.
Thus frees water in concrete, in other word, free water/cement ratio of mix depends on rate of absorption during mixing and unlike normal weight aggregate concrete, depends not only on moisture content of aggregate particles. So selection of water/cement ratio in designing mix proportion is quite difficult.
But when suitable coating over the surface of lightweight aggregate is provided and selecting relatively round and low absorbing aggregate, normal mix selection is found often satisfactory. The key is to limit absorption. Otherwise instead of water/cement ratio, the basis of mix proportioning is considered cement content of the mix.