You need to take a statistics course. And I would not suggest you gamble.
A 50% chance is just that. You roll the dice and take your chances. 10 failures out of 10 is not indicitive of something other that a 50% chance (I am theorizing here so 50% is not what I think the chance of a TS bow is).
Having dexterity enhanced, and fletching high, will improve chances, but still a chance. So do your best with what you have.
Serutan, 24th Season Ranger of Tunare
Veeshan Server

Bottle of Clockwork Oil
Clockwork Spider Abdomen Plate
Clockwork Spider Eye Lens
Clockwork Spider Leg Section
Clockwork Spider Mesh
Clockwork Spider Thorax Plate
Crude Defiant Chain Bracer
Crude Defiant Chain Leggings
Crude Defiant Cloth Pantaloons
Crude Defiant Cloth Robe
Crude Defiant Cloth Sandals
Crude Defiant Cloth Sleeves
Crude Defiant Leather Tunic
Crude Defiant Plate Gauntlets
Crude Defiant Plate Greaves
Crude Defiant Plate Vambraces
Drop of Grease
Mechanical Light Emitter
Micro Servo
Rusted Blackbox
Scrap Metal
Size 4 Gizmo