That's the formula.

Each move has a certain percent. flamethrower for instance has 100%. That's what the A base represents.

Accuracy is how many stages raised or lowered on the attacking pokemon, and evasion is the same, but for the defending pokemon.

flamethrower 100 acc. 100*1=100%. no calculation for that.

Flamethrower with 1 stage of lowered accuracy.

-1 stage= 1*.75=75% chance

Thunder with 1 stage of lowered accuracy.

.75* .70=0.525= 52.5% chance.

the evasion/accuracy chart is as follows:

Stage -6: (3/9) 33.3%

Stage -5: (3/8) 37.5%

Stage -4: (3/7) 43%

Stage -3: (3/6) 50%

Stage -2: (3/5) 60%

Stage -1: (3/4) 75%

Stage 0: (3/3) 100%

Stage 1: (4/3) 133.33%

Stage 2: (5/3) 166.67%

Stage 3: (6/3) 200%

Stage 4: (7/3) 233.33%

Stage 5: (8/3) 266.67%

Stage 6: (9/3) 300%