If you don't have any plants, as long as nitrates are under 40, you should be fine. Normally when you don't know (it's between), you usually round to (in your case) 15. You can't really make a good estimate of any precision beyond that.
It's possible the .3 nitrites was the cause, but normally that amount of nitrites would cause stress rather than death. I'd put the % of deaths somewhere between 30-60% depending on the fish, so the sooner you buy them when they ship, the more likely you're going to get one of the fish that were already going to die. (usually beyond anything we can do, shipping losses are kind of a given, except in maybe the more hardy cories and danios, and if you lucky tetras). I try to buy fish the day before the new shipment comes in so the fish have a week to "settle down". Sometimes when I forget, (a few times I bought it on the day of the shipment), I got close to 100% losses (these were cardinal tetras). When bought the day before, it's closer to 0% losses. Depending on the store, and the quality of the fish, I may buy on the day of shipment, as they get rarer fish, and generally they do pretty well.