Shouldn't the plug an play hav e2 connections for the front? One on under each side bow cushion? If not that is pretty lame. I don't have any experience weighting an x45 but a friend use to have one that we surfed. That boat needs lots of ballast to get a really good surf wake. If you are planning on surfing I would get atleast 750's or whatever the biggest bags you can for the rear and whatever the biggest bags there are for the front. I would think you could get 350's or so under those front seats. I would think these 2 options would be your best.
Not sure this will fill up all the way but may come somewhat close.
With 750's or so in the rear you have the start a good surf wake and a good start for boarding you would fill the front and maybe try the rears about half full. Play with the trim tab. Then decide if you need more weight in front or rear? I would probably keep an extra 500# sack or maybe 2 to use if you want to get more weight in the front for boarding and more weight to side for surfing.
Now if you don't surf and don't plan to I would probably just go with 350-450 sacks in the rear so you can just fill everything completely full.