Are you using table sugar (cane sugar) or corn sugar (dextrose)? How much carbonation do you want in your beer? Belgian styles usually have high carbonation levels compared to other styles of beer.
5 ounces (140 grams) of corn sugar would be a good starting point. If you use table sugar then you probably want to increase that to about 5.5 ounces (155 grams). this would give a little more carbonation than most styles of beer, but a little less than the amount that is "correct" for a Belgian. I would not use too much more sugar than that, though, unless you use special "Belgian" bottles or champagne bottles. These bottles can resist higher pressures without breaking. The high pressure of carbonation can break regular beer bottles if you use too much sugar.