الجواب حامدا و مصلیا
1. It is necessary to specify and make Niyyath (intention) at the time of giving any kind of Sadaqa. NIyyath is the name of the intention and the firm idea that comes in one’s mind. If a person intends to spend some money in building a certain mosque and later, while paying the money, even if he could not tell anything in words still he gets the complete reward. However if he utters it in words it is much better.
2. Yes it is permissible and correct to make intention of doing Esal e Sawab (Bestowing Rewards) to a single soul or all the Mumin males and females and the reward of the complete Sadaqa will be received by the intended soul and also to the one who did the Sadaqa.
3. There is no particular amount or a percentage of salary is specified for Nafl Sadaqa in Shari’a. May give in Sadaqa whatever amount is possible to the extent of one’s capacity. However it is better to give in Sadaqa such an amount once and for all so that the receiver need not have to ask from another one for that days requirement.