calculation of sulfamethoxazole -trimethoprim dose in PCP (simple way)

Recommended dose based on trimethoprim 15-20 mg/kg

1. 20mg (trimethoprim)X body weight = dose / day
2. 20Xwt/80 = bottles /day
3. 20X wt/80/4 = bottle/day
4. bottle/day / 480 = dose/6hrs

off course not the above one but see next,

Simply , mathematically

Wt X 30 = dose/6hrs 

then round to nearest figure to avoid wasting bottles = 1440 mg, 1920 mg, 2300 mg /6 hrs