Ok, Evasion +1 means a 20% boost. But how can i calculate the chance of the move hitting? My poor knowledge in maths says a 100 acc move will have 80%, but serebii says it would be 75% and bulbapedia says it would be 83%. Can you guys help me? Thank you :)

Evasion is calculated as follows: Evasion is calculated as a fraction starting with 3/3. For every stage increase in accuracy or drop in target evasion, the numerator increases by 1, and for every stage increase in evasion or drop in target accuracy, the denominator decreases by 1. For example: accuracy starts at 3/3, or 100%. After a Double Team, the denominator increases by 1, so the result is 3/4, or 75%. After another, the result is 3/5, or 60%. If the opponent then used Hone Claws, the result would be 4/5, or 80%. Sand Veil is NOT a +1 Evasion boost, it's just a raw 20% accuracy decrease.