Sorry for the delay. Mike gave you a good explanation on how to use purlins.
As a practical matter, once you have calculated the drop at the far end of your floor, rip the first purlin and install it against the lowest wall. (1X3 stock and/or 2X4'stock should work well) I would then use a very straight edged piece of board, cut to the length of the highest level to the lowest level and set it on the floor bridging the entire space. Now you can easily measure the gap to the existing floor at 12 to 16 inch increments in order to give you the thickness of each successive piece of purlin.
You will also want to install a perpendicular nailing strip between the purlins at 8 foot on center, alternating as you would when installing the plywood. This is just a little extra support for the butt ends of your new subfloor. Cut these to the smaller purlin dimension, that will be fine. So what you will have created is a grid to lay your underlayment on.
Speaking of underlayment, I would highly suggest you use a tongue and grove style 3/4 inch underlayment. Do not leave gaps in this underlayment. Snug them together well. This will make the absolute best, stable platform for your new hardwood.
Also, to assure a nice solid quiet floor, lay a bead of construction adhesive on each purlin before you install the underlayment. Secure the purlins and underlayment with 2 1/2 to 3 inch ring nails or decking screws.
When you install your floating floor, be sure to use a foam backing layer, unless the product you have selected has a backing preinstalled.
If you need specific questions answered, see my profile for my e-mail address. This process is actually quite easy and doesn't require any advanced skills. As long as you have a decent table saw and a feather board, you can rip out purlins in a snap.