The rule is to follow the manufacturers instruction when it comes to ratios, however sometimes the dispensing can go wrong. In the exam, it is important to get the mix right, but if you get it wrong, you should also be able to explain why it may have gone wrong, and if you have time, have another go.
Alginate - are you using the correct scoop and measuring cup? have you shaken the alginate box? when you scoop the powder are you pressing it down once with the spatula then levelling it? is the water exactly on the line , check on a flat surface.
Cement, GIC, Eugenol etc - again, follow the recommended measurements , but remember, sometimes the dispensing can go wrong. With the powder, divide it into sections with the spatula, then mix it into the liquid bit by bit, if the mixture is too wet, don't use all the liquid, they want to see that you know the consistency should be as well as being able to follow instructions.
I hope thats helpful?
Good luck!