Well first of all, you will be doing fairly well even if you screw up, LOL. You can use those 2x4 trays, but IMO it would be allot more productive to be using 4x4 or 3x3 trays under each 1K light even if the 2x4 tray gets full it still won't fill up a 4x4 space inless you are going to be doing some extreme trading.
1 other thing, I would really try to be very sensitive to the strength and directions of the Purple Max and Gravity because if used improperly they can fry your crop no questions asked so get as much related info as possible on the application and don't get brave when using them. Perhaps you have used them before and I am wasting my breath, but if not, just be careful.
Other than that you got it cracking and happy growing. Look forward to your grow.
Adieu