What percent of your blood can you lose before you need a transfusion
Typically transfusions are indicated for hemoglobin &lt; 7 g/dL (for adults normally: 12-15 grams per deciliter [1]) or &lt; 8-10 g/dL if there is cardiac disease where diminished oxygen carrying capacity cannot be compensated by increased cardiac output. 

The answer above is theoretical because there is no accurate way to measure blood loss in the clinical setting - it's usually a mess. 

The most useful parameter to assess intravascular volume in the immediate term is blood pressure. Again, the actual number is variable but a 30% drop from baseline in the acute setting is bad. 

Another less acute indicator of hypovolemia is urine output (normal is .5-1 ml/kg body weight/hr, dropping below ~35 ml/hr therefore is an indicator of hypovolemia.)[2] Hypovolemia is often corrected with sterile saline fluid and adding packed red blood cells only as needed to keep hemoglobin in the above required range, unless someone is actively bleeding (trauma, bleeding ulcer, etc.) in which case "whole blood" (or packed red blood cells, fresh-frozen plasma and platelets) is given.[3]

[1] McPherson: Henry's Clinical Diagnosis and Management by Laboratory Methods, 22nd ed.
[2] Oliguria &amp;  Volume Depletion
[3] Fluid Resuscitation and Volume Assessment