There is a 40%/30%/5% chance on N/H+VH/OK+DW that chemicals for 3 bags of meth spawn in the meth lab. If this chance fails, there are ingredients for 1-2 bags.
There are 19 crates in the basement. Upon opening a crate, the game looks at all available crate contents and randomly chooses one. Available is anything that hasn't been spawned 3 times yet. A crate contains one of these 6 things: nothing, missiles, vodka, coats, cigars, chemicals. If there isn't anything available anymore, the first item in the list (nothing) will spawn.
One of the crates ignores all availability requirements and has equal chances to spawn all 6 items regardless of how often they have spawned already. The item spawning in this crate still works against the availability for the other crates however. The crate is located opposite the bar-code scanner, about 20 m away: There are 3 crates, a long one to the left and 2 flat ones to the right and center; the one in the center it is.
Another crate obeys all availability requirements except the one for chemicals. If nothing is available, it will always spawn chemicals. That crate is located in the corner to the left of the bar-code scanner (the flat crate).
By opening the crate that may contain anything first and the crate in the corner last, the basement will always yield enough ingredients for 4 meth bags. The following screenshot marks the 2 crates in question: