Forget the WPG rule. It all depends on what type of plants you want for the lighting you'll need. For example of I have all low light plants ,anubias, java fern, a crypt and some amazon swords and run only one 48' T8 bulb and have good growth. A double strip T8 or single T5 HO will allow you to grow medium light plants. People don't realize you don't need as much light as you think to grow plants. If you're going with higher light plants then I would suggest a dual T5 HO but then you'll be forced to run a co2 system will all that light or you will get only one thing to grow and that's algae.
Always start off with as minimum of lighting as you'll need for the type of plants you want. You don't need all these fancy setups like you may think. Having a successful planted tank is all about balance....ex. How many fish you have, how much lighting, fertilizer, how long you leave the lights on and all that. Without balance you're going to run into problems along the way. It's kinda trial and error. Every tank is different. Certain plants they may be tough to grow may grow fine for you, then you may struggle with some that are perceived to be easier .... Happened with me and I'm sure many others.
Also would like to throw in that water parameters play a big role too. Some plants prefer soft or hard water , example jungle Val prefers hard water and if yours is soft you can pretty much forget about growing that.
. Look up PAR.
