Backtrack= OP or just annoying? (Warning: Potential Rant ahead)

Here's the simplified information:

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This is how it works.

Take that +rep and my respect!!!

Total evasion percentage = [ 1 - (1 - first source of evasion) x (1 - second source of evasion) ... x (1 - n-source of evasion)] x 100

Let's say PA's Blur has a 25% chance to evade an attack for easier comparison with void's backtrack

If both have a Butterfly in their inventory :

Pa's total evasion percentage will be,
[1-(0.65)x(0.75)]x100 = 51.25% chance to evade an attack.

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Since backtrack only take place after all normal evasion are not triggered.
Normal evasion percentage = 35%
backtrack evasion percentage = 25%

So,
Normal evasion procs = 35%
Normal evasion failed, but backtrack procs = 65% x 25% = 16.25%
Both normal evasion and backtrack failed = 65% x 75% = 48.75%

Thus, Total chance to evade an attack = 35%+16.25% = 51.25%

Conclusion : Pa and void has the same total evasion percentage if they both have a Butterfly in their inventory.

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What if there are 2 Butterfly?

Pa's total evasion percentage,
[1-(0.65)x(0.65)x(0.75)]x100 = 68.3125% chance to evade an attack.

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Void's total evasion percentage,
Normal evasion percentage = [1-(0.65)x(0.65)]x100 = 57.75%

So,
Normal evasion procs = 57.75%
Normal evasion failed, but backtrack procs = 42.25% x 25% = 10.5625%
Both normal evasion and backtrack failed = 42.25% x 75% = 31.6875%

Thus, Total chance to evade an attack = 57.75%+10.5625% = 68.3125%

Conclusion : Stacking any evasion will have a diminishing effects, including backtrack.

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backtrack is not an evasion, I don't know how it works in DotA 2 but its a triggered post-heal after an attack/nuke in DotA 1 (WC3), which means in DotA 1 if void has 1 hp, he will always die if he is attacked or nuked since heal cannot take place if he is dead.