HOW much people spend on food, fuel, medicine and entertainment will be the yardsticks to determine whether social security assistance is high enough. A study by the Hongkong Council of Social Service, to be launched next month, was prompted by the Legislative Council welfare panel, which approached the council because of criticisms of low public assistance. The aim is to determine whether the amount allows recipients to lead a ''modest but adequate'' life, according to the council. A Comprehensive Social Security Assistance Scheme began this month to replace the old system, which was introduced in 1971. The Government says the average payment for a single person is $2,000 - an increase of $320 or 19 per cent since the beginning of this month. It maintains the money covers the ''essential requirements for food, clothing and fuel''. But some groups are sceptical about the scheme and accuse the Government of ''playing with figures''. The council will interview 700 households living on public assistance. The study is expected to take a year and the findings will be given to the Legco panel.