I used to know this pattern, but I lost it and never bothered to figure it back out again. It works like this. Each Robot Master has ten possible dots in the password grid: five for if you have defeated him (and therefore have his weapon) and five for if you haven’t. The reason there are five is because the numbers change depending on how many Energy Tanks you have.

To create a password, all you need to do is pick a column for how many E-Tanks you want, then fill in the appropriate dots. For Robot Masters, the first digit defeats him and gives you his weapon, the second leaves his stage available. Obviously, you only want to use one of the two numbers separated by slashes.

E-Tanks: None One Two Three Four
E-Tanks A1 A2 A3 A4 A5
Air Man E3/D2 E4/D3 E5/D4 B1/D5 B2/E1
Bubble Man D1/C3 D2/C4 D3/C5 D4/D1 D5/D2
Crash Man C5/E2 D1/E3 D2/E4 D3/E5 D4/B1
Flash Man C1/E4 C2/E5 C3/B1 C4/B2 C5/B3
Heat Man B2/D5 B3/E1 B4/E2 B5/E3 C1/E4
Metal Man E5/E1 B1/E2 B2/E3 B3/E4 B4/E5
Quick Man B4/C4 B5/C5 C1/D1 C2/D2 C3/D3
Wood Man D3/B5 D4/C1 D5/C2 E1/C3 E2/C4

Trivia: Visitor warmCabin reports that the dot in row A (for E-Tanks) is technically optional. If nothing in that row is filled in, the game assumes A5 (four Energy Tanks). So you can place a stray dot somewhere else in the grid which is basically ignored.

This data was originally from John Snow and later supplemented by (*gasp*) Nintendo Power.