So, out of curiosity, I tested Hustle, Hone Claws and Lax Incense...
Premise: Hustle 80% accuracy, 1 x Hone Claws = 4/3 accuracy, Lax Incense 95% accuracy (or 90%)...
I used a Tornadus holding a Lax Incense with a partner Hustle Durant with the moveset Metal Claw, Bug Bite, Hone Claws & Iron Head (later with Giga Impact) in a wild double battle.
Tests:
1. Hone Claws Hustle Metal Claw (95 acc) vs Lax Incense:
a) 95 x 0,8 x 0,95 x 4/3 = 96 acc
b) 95 x 0,8 x 0,90 x 4/3 = 90 acc
c) 95 x 0,9 x 0,95 x 4/3 = 100 acc
d) 95 x 0,9 x 0,90 x 4/3 = 100 acc
Result: After 1 x Hone Claws, Tornadus avoided the 50th and the 61th Metal Claw attack from Hustle Durant targeting it.
2. Hone Claws Hustle Bug Bite/Iron Head (100 acc) vs Lax Incense:
a) 100 x 0,8 x 0,95 x 4/3 = 100 acc
b) 100 x 0,8 x 0,90 x 4/3 = 96 acc
Result: After 1 x Hone Claws, Tornadus avoided the 53rd, 73rd and 74th Bug Bite/Iron Head attack from Hustle Durant targeting it.
Lax Incense can´t be a 5% modifier nor a 6,25% modifier (1/16) if Hustle is a 20% modifier.
3. Hone Claws Hustle Giga Impact (90 acc) vs no hold item:
a) 90 x 0,8 x 4/3 = 96 acc
b) 90 x 0,85 x 4/3 = 100 acc
Result: After 1 x Hone Claws, Tornadus avoided the 33rd Giga Impact attack from Hustle Durant targeting it.
Hustle is indeed an (at least) 20% accuracy modifier, unless 1 x Hone Claws is a lower than 4/3 accuracy modifier.
EDIT: I should probably test Hone Claws+Hustle+Metal Claw to see whether Hustle is 80 acc (it would never miss) or 75 acc (it would miss 1 in about 20 tries)
I´ve also tested Fiery Dance % of raising the spA, various sites list it as 50%. Well, coin flips are always difficult to test. I got:
6 in 10
4 in 10
6 in 8
6 in 16
6 in 13
a total 28 spA boosts in 57 fiery dances, so my tests were pretty accurate with a 49% probability.
Fiery Dance has a 50% chance to raise spA.
EDIT: Also tested Hone Claws + Hustle + Metal Claw and Durant hit 112/112. Hustle is definitely a 20% modifier.
To test whether Lax Incense is a 10% modifier (x0,9), I´d have to use Wide Lens vs Lax Incense with a 100 accurate attack, whether there´s a 1% chance to avoid an attack. Maybe I´ll do this test later.
Also tested Sacred Sword yesterday, it bypasses foe´s Double Team evasion boosts but will miss against Snow Cloak (damn).