THE Urban Council has been warned to do its sums better after major inaccuracies were revealed in budget estimates for the past 20 years. Figures obtained by the South China Morning Post show that the difference between the estimates and results ranged from $21 million in 1973/74 to $759 million in 1991/92. Either the surpluses were underestimated or deficits overestimated. A former chairman of the council's finance select committee, Walter Sulke, said: ''The expenses of the Urban Services are always overestimated. In the end, when they are forecasting a deficit, they will have a surplus. ''I tried very hard, when I was the chairman of the finance select committee to get them to make their estimates accurate. ''But I think civil servants always like to put in a little extra. They don't want to be caught out afterwards.'' He added: ''I'm sure they are trying to get more rates out of the Government. Of course, I can't go to the Government to ask for more money if I have a surplus.'' The Urban Council takes three percentage points of the 5.5 per cent annual rates in urban areas. The rest goes to the Government. It is understood that the council will soon negotiate with the Government its financial requirements based on its new five-year forecasts, while the Rating and Valuation Department is now reviewing the rates. The forecasts show operational deficits of $325 million, $195 million and $175 million at current prices over the three years from 1994-95. Mr Sulke said overestimating expenses was as bad as underestimating them, because the money could have been allocated for more appropriate purposes. He said overestimating encouraged waste by ''asking for money not needed and spending on things not necessary''. Mr Sulke said it should be the councillors' responsibility to monitor more closely the budget prepared by Urban Services Department officials. Secretary-General of the Association of Cost and Executive Accountants in Hong Kong, Patrick Chan Chau-ming, said any estimate having a deviation of more than 30 per cent would be regarded as inaccurate. He warned that the council should correct the problem or its credibility would be damaged. But the current chairman of the finance select committee, Brook Bernacchi, said that accusations that the council had overestimated its expenses were incorrect. A council spokesman said: ''Experience has shown that such forecasts are normally reasonably accurate, bearing in mind the many economic variables that can affect any forecast.'' Finance Select Committee member Stephen Lau Man-lung said these uncontrollable factors affected any budget. ''For example, any delays in any capital project will lead to savings,'' Mr Lau said. In the years 1981, 1982 and 1991, the council recorded a surplus despite the deficit forecast, and a spokesman said this was because of ad hoc grants and rates re-valuation.