I just bought 2 lumber tarps, 8' sides it has 3 rows of d rings. What's the best length of bungie or rubber strap, or combination of straps :? :?

Better to buy them longer than shorter. I have 200 21" bungee cords. If they are too long you can go under the rub rail then back up to another D ring or you can go under the rub rail, stretch it then hook onto a rub rail cross member. If you buy them too short there isn't much you can do other than hook 2 together.

I prefer 21" bungee's.