Its pretty good, but not perfect. At doom I first thought it was random when it would let damage hit, but I do think I've found the issue. If I eat or drink a potion just before something is going to hit then it won't pray. I'm not sure if this is intentional as I guess you might not be able to do that normally, and could be seen as fishy by jagex but it looks like a bug to me. Also it does seem a bit punishing as I do think you would be able to do this playing normally, as its based on ticks, and if you eat and pray in the same tick then you'd be fine. I think thats the issue anyway. Its not every single time but its a good amount of the time.