Ursoc Major HP% Bonus caps at 30%. you cannot get 100% like the above posters is saying.
With no haste (1.5sec GCD), you can get 16.667~ abilities in. Assuming 1 of those is a thrash.. and UM stacking 2% (to 30% cap) every attack.. you would need 15 multistrikes out of 15.66667~ possible attacks.
edit - UM is 2% gain per Multistrike of Lacerate/Mangle, or Lacerate tick. This lasts for 25 seconds and caps at 30%. and rolls like mage ignite/lock and load explosive shot.
With 25% haste (20%+5% raidbuff), dropping your GCD to 1.25 (My math may not be right on the haste->GCD stuff.. but bear with me)
You would get 20 attacks, 1 of which is thrash.. Leaving 19. 15 / 19 = requires 78% multistrike to maintain it at cap.