The knack for cup & cone hubs is to actually slightly overtighten the cone, because it will loosen back off a bit when you tighten the locknut against it.
Try this: Hold drive-side locknut with 17mm spanner & tighten off side cone with 15mm cone spanner until the bearings are tight, but you can still move them. With another 17mm spanner ('cos you're still holding the other end), tighten the off side locknut a bit, then with your 15mm cone spanner, tighten the cone against the locknut. It will back the cone off from the bearings, but you might need a couple of goes to get the feel of just how much you need to overtighten it at that first stage.
Just ride the bloody thing and enjoy yourself!