What is the percentage that must be given from fixed income if the money does not reach the zakat threshold, and should it be intended as zakat or charity?

Zakat is obligatory on intrinsic wealth—such as gold, silver, and their equivalents in banknotes—provided that a full Hijri year has passed and the zakatable threshold (nisab) has been reached (which is equivalent to 85 grams of gold). If you save the nisab and a year passes, then zakat at a rate of 2.5% becomes obligatory on you. Newly acquired wealth of the same type is zakatable along with the original amount during the year. However, if you have not saved a complete nisab, or if you spent it before the year passed, then zakat is not obligatory on you. Voluntary charity (sadaqa) is recommended, and one can give any amount, and increasing it with sincerity increases the reward. As for the intention, if it is for paying the obligatory zakat, then the intention to perform zakat is required; otherwise, the intention of general charity suffices.