Sailboarder has it right!
On an UltraCat or any more traditional longboard, with the rear planing surfaces extending well forward of the mast track, moving the mast foot back will not really help very much with getting planing, but it still applies.
Best to take a string, stretch it over the bottom of your inverted Ultracat and find out where the flat surface ends.
Then you can take a couple of little electrical tape "arrows" and mark these positions near the rails on the top of your board.
Then you will know where the "rocker transition" actually is and you can use the arrows as a guide when you are sailing.
When planing, having as much of the board as possible, out of the water is always faster, until you reach a speed where control becomes more important, then you can move the mast foot forward a little to get the best balance between speed and control.
To get planing, yes you want the board very flat in the roll (rail to rail or athwartships) axis. You also need to get the bottom of the board (the planing surface (s) inclined so they are higher at the front.
This is called "positive pitch angle". Your weight placement has more effect on fore and aft (Pitch) trim
than the mast foot does but moving the mast foot back also helps you to move your weight back.'
If you really want to trim your board out to plane well (even in marginal conditions) you need to get enough speed so you can get back and into the rear (planing) footstraps.
I'm a fairly light guy (160 lbs./ 72.6 Kg. back in the day) and I remember the first time I was fully planed out and back in the rearmost straps on an F2 380 long board. Wow, it seemed like about 11 feet of that 12 foot + board was out of the water. Not so easy to control, but pretty fast.
So, to plane well, flat tirm side to side, positive (pitch trim) fore and aft.