THE leading proponents of a flat tax are economists Robert Hall and Alvin Rabushka who suggested levying a 19 per cent flat rate on salaries. There would be no deductions, though Hall and Rabushka suggested exemptions of $16,500 for a married couple and $4,500 for a child. Capital gains, interest, and dividends would be exempt from taxes. Businesses would lose many deductions, although investment spending would get an immediate 100-per cent first-year tax write-off. Companies would pay a 19 per cent rate on net profits after deducting wages, salaries, pensions and cost of goods and materials.