什麼叫主水平基準

it's the level +0.00m

generally it suppose to the sea surface level

let say
mid levels area like caine road is around +90.00m that's above sea level by 90 meters

and it reduce from +165 to +100 means the height constraint of the site become strigent.

the plot ratio also reduce to 8.6(actually 8.6 is quite high)

so the total gross floor area becomes less

all the above reducing the building mass which is a good news cause the final buildings' height and size can be controlled.

P.S. the mass has to consider site coverage as well

hope can solve ur questions