THE least controversial of Governor Mr Chris Patten's constitutional proposals - the Boundary and Election Commission Bill - was passed yesterday despite objections from two pro-China legislators. But the United Democrats' (UDHK) attempt to amend the bill to narrow the margin of permissible deviation in the population size of constituencies from 25 per cent to 15 per cent was defeated by a 16-27 vote. Mr Tam Yiu-chung and Mr Philip Wong Yu-hong considered it inappropriate to endorse any proposal relating to the 1994-95 electoral arrangements when China and Britain were discussing them. The bill, which provides for the setting up of a commission responsible for making recommendations on demarcation of constituency boundaries, arranging and overseeing electoral procedures, was eventually approved by a 41-2 vote. Proposing an amendment to the bill, UDHK chairman Mr Martin Lee Chu-ming said it was of paramount importance to narrow the deviation margin in population quota to minimise the chance of gerrymandering. ''It must be obvious to all that every Hongkong citizen's vote should have the same weight. It must be equally obvious that all electoral constituencies have equal populations as far as possible,'' he said. The Government's proposal of a 25 per cent margin could result in a discrepancy of as much as 66.66 per cent between two adjacent constituencies, he said.