Is it permissible to give a secret charity on behalf of one's parents without precisely calculating the amount, or is secret charity exclusive to the giver himself?

Secret charity is not exclusive to the one who gives it; it is permissible to give it on behalf of one's parent, whether alive or deceased. The giver of charity will receive reward and recompense for it, and this is a form of righteousness. Allah may combine for him the bounty mentioned in the Prophet's (peace and blessings of Allah be upon him) saying: "Secret charity extinguishes the Lord's wrath, and maintaining ties of kinship increases one's lifespan."