You are absolutely right about having new additions tested for FeLV. I lost one of my beloved cats to the disease when I brought in kittens who were supposedly tested for it. He was an adult and had been vaccinated for the disease. I lost all 3 of those cats.
But you need to be aware of the chances for spreading it. It is not spread through casual contact. Cats typically get it through fighting or mating. If they get it through casual contact (mutual grooming, shared food and litterboxes), it is after prolonged contact. My boy got it from the kittens after 6 months of very close interaction. I don't think there has ever been a confirmed case of a cat picking it up by a human carrying the disease from household to household.
If you cat has been vaccinated, the odds that they will contract it will go down, and studies indicate that about 10% of these cats in constant contact with FeLV cats will contract the disease. Cats that have not been vaccinated have somewhere between a 25% and 33% chance of catching the disease. In my case, I had 13 cats in the house. 2 of them (the kittens) came into the house with the disease. Of the other 11 cats in the house (all vaccinated), only 1 picked it up, even after living together for 6 months.
You said that 1/3 of all cats will die from the disease. I'm not sure I follow you on this. Once a cat is exposed, they can test positive on the Elisa test. This does not mean that they have the disease, it only means that they were exposed and they are trying to fight it off. If they can't fight it off (the odds above), then it goes into their bloodstream and the only way to validate this is through a more invasive IFA blood test (full vial sent to a lab). All cats that contract it will die. Kittens usually by the time they are 18 months old and adults can live longer, sometimes years. It will shorten their lives, but all eventually succumb to the disease. It is 100% fatal.
You need to take FeLV very seriously. But also be aware of how it is transmitted and don't expose your cats to those situations.