Solid Hardwood, Engineered and Laminate Flooring - Efficient way to level plywood area

I am installing laminate flooring in my house, and am having a very hard time efficiently leveling the plywood base floor.

I have tried 2 techinques:
1) I obtained a relatively flat 8' 2x4 and laid it at 1' increments along the floor and marked the low spots. I then rotated 90o and redid this.
This failed because of the uneveness of the 2x4. It wasnt level to begin with, and I eneded up with a nonsensical mess

2) I tried the same thing with a piece of string that I tied very tightly. It was too difficult to tell where the floor was sagging this way.

I tried shopping aroun dfor a 10' aluminum screed (as I had seen on this sight) but no one at my local hardware store had any idea what that was or where I could get one. Any suggestions?

I have roofing paper and a sander.
Thanks for the help!

If you can't tell with a string, then it is not worth fretting over, lay the floor. Laminate floors do not require an absolutely flat floor.