How accurate are nuclear stress tests? if they only come up positive with a 65% blockage then is that a reliable test for ischemia?

They are much less: Invasive (and safer) than a coronary angiogram. They detect significant coronary artery stenosis by differential blood flow that occurs between resting and cardiac stress (exercise or drugs). It does take a certain level of stenosis to detect the regional decrease in blood flow, but that same considerable stenosis is required in order to get symptoms (chest pain, or angina).