The short answer is, no, you can't just add the preload to the pushrod length; there's geometry involved that won't let that plan work. To get your pushrod length, you should set valve lash at where you will normally run it, run the valve through a few full cycles with a solid lifter until you have a clear wear pattern on the valve stem tip (I use an erasable whiteboard marker on the tip since it wears off easily), and adjust the pushrod length and repeat the process until you see the desired wear pattern on the tip. For more detailed instructions, there are plenty of free tutorials online through valvetrain manufacturers and performance magazines.
