Use hub or tack for grade??

Probably on whatever sticks up the highest. But there is no solid rule on this one way or the other. If truly precise elevations are needed you shouldn't be using hubs and tacks anyway.?ÿ

Double-headed nails will give you a high point and something for the form builder to hook his measuring tape to.

Set top of form at grade or known offset to grade and set tack or construction nail for line

Always use the highest point on the hub because thats what the contractor will use. If elevations are critical you should be taking care to set your hubs level and the tack flush anyway and it wouldn't be an issue, or use a dedicated benchmark that is something more stable than a wooden hub.

I agree

You should put your rod on the nail. If you put the level on the nail it will be hard to see through it.?ÿ