The admixtures water reduction capacity is dependent of the compatibility of the admixture with the cement, thus the % could be variable and needs to be determined by testing. Otherwise the admixture industry give in their catalogs some general information. 

I am agree with Luiz Antonio Pereira, The exactly quantity of water that could be reduced with the superplastizicers admixtures only is possible to  know making some tests in the laboratory with the disponible aggregates and cement. May be different with different aggregates and cement. The admixture industrie only will offer you an aproximated value or an interval of water reduction.

M. C. Nataraja. I too agree with the answers given by Luiz and Juan. Basically the water reduction capacity depends on the action of admixture namely its electrostatic repulsion and steric hindrance capabilities. With new generation admixtures (hyperplasticisers), about 35 to 40% reduction of water is possible. Simple flow table test can be conducted to find this reduction.

 I agree with the answers of my colleagues, the ability to reduce water also depends on the type of cement and the absorption capacity of the aggregates. So, only laboratory tests can be conducted to find this reduction.

Article attached in French language, may be useful for you

As an addition to the answers above. I would like to add that the influence of the admixture also depends on the time between adding the admixture and the moment of measuring the workability. Also the state of the aggregates (wet or dry) at the moment of adding the admixture makes a big difference.

I have added a link to one of my papers concering this topic.

https://www.researchgate.net/publication/280608988_Assessment_of_water_reducing_admixture_in_concrete_with_recycled_aggregates

Conference Paper Assessment of water reducing admixture in concrete with recy...

Since the objective of adding the water reduction admixture is to increase the workability of the concrete mix of low water contents, workability tests (eg slump tests) would be the ideal tests to carry out to compare the effectiveness of the admixtures

Admixtures having lot of different various Properties in composition with Concrete , Admixture Type-A, Type-B, Type-C, Type- D ,Type-E, Type -F and Type- G these types are  giving various Properties with respect to different type, Through this can easily differentiate with various Parentage of Dosage in each Types . like water deduction capacity , high strength , workability , various set to finish Surface of concrete.    

Rather than trying to calculate the water reduction capacity decide on the Water / cement ratio then when you do your trial mix you add the super plasticizer till you achieve your desired slump (workabitity) different brands will give you different workability for same dose rate.

I think that all the answers have a pretty good contribution for your need. I just want to add two aspects that I think you should have in mind. The first one is that some admixtures changes the rheology in a very different way than only add more water. So, it is possible to interfere in different ways in the yield stress and viscosity. So, be careful with the type of test you are going to chose in order to evaluate workability. The second aspect is linked with the difficulties that some mineral admixtures provide to analysis. Especially when high content of slag furnace is added to the mixtures, the variability of the response is greater using these admixtures because every water reducer acts in the surface of fine materials and slag furnace could increase variability.

I think that everything has already been added by the contributors above. I may just illustrate a little bit. For example, the water reduction capacity will be greatly influenced by the aggregate gradation (both coarse and fine). It is fine aggregate that will have more influence and so fine to coarse aggregate ratio should be taken as an index. Of course, the basic binder Ordinary Portland Cement or Portland Composite Cement (having different fine composition like lime, slag or fly ash etc.) shall have bearing upon the workability with added admixture.

So, as advised by others its better to determine on site water reduction capacity of admixture with all the constituents of concrete mixes available at hand.