To calculate how accurate the ball will be, you look at the Pokemon's weight in the pokedex entry. The numbers vary by game though:
-20 if used on Pokémon weighing less than 220.6lbs(GSC) or 451.5 lbs in HGSS
+00 No modifier if used on Pokémon weight between 220.6- 441.0 lbs (GSC)
+20 if used on Pokémon weighing between 451.5 lbs and 677.3 lbs
+30 if used on Pokémon weighing more than 661.5 lbs (GSC)
+30 if used on Pokémon weighing between 677.3 lbs and 903.0 lbs (HGSS)
+40 if used on Pokémon weighing more than 903.0 lbs (HGSS)
The numbers refer to part of the catch rate. All you really need to know is that the obvious "the heavier, the better."