Hmmm...It's all up to luck, most of the time. I caught my Suicune in Route 6, but that doesn't mean it can't appear elsewhere. The best way to meet a fleeing legendary is to travel between Routes. Try going between the Routes 16, 17 and 18. Don't use Repel is what I'd advise. Check the grass in each route a few times(2 or 3 encounters with Pokemon should be enough) before moving on. Suicune is relatively common in the routes it appears in
Also I'd advise that you try and stock up on some Timer Balls. They become just as good as Ultras after 40 rounds, which means if it still hasn't been captured by then, you have a few balls to spare.