Is the mother's (wife's) share calculated from the total inheritance after deducting the grandmother's (deceased's mother's) share, or from the basic total inheritance?

Your paternal uncles and aunts have no right to dispose of their mother's money by gifting or relinquishing it, because it is not their money. It is not permissible for anyone to gift another's money without their permission. Your grandmother is considered to be in the status of an insane person who is interdicted for her own benefit. It is not permissible for her guardian to gift her money or relinquish it. Your grandmother's share remains preserved for her. And the wife receives one-eighth of the entire inheritance.