If you are picking an Affinity color and you can afford it, why not use Aura Matte? It is a significant upgrade from the Regal Matte in my opinion. If you use a matte finish on a ceiling, you may notice flashing, lap lines from the roller, drywall seams and flaws, etc. It is really the reason we try to use dead flat paints on ceilings because all the light goes up there and you can see every flaw. The Aura Ceiling Paint is a good dead flat option.

As for diluting color . . . You cannot simply cut the colorants without understanding the base color and what the colorant is. Example. I have a can of paint here that has 4.25 / 32 Y3 Oxide Yellow 5 / 32 S1 Black 12.125 / W1 White 2.5 / 32 R3 Oxide Red in Base 2 If I cut the formula in 1/2 the white also gets cut in half and I will end up with a color that is darker, not lighter than the original formula. The computer systems are literally doing the math, but not all colorants are equal in strength - red it more transparent than most other colors, yellow is weaker than many other colors, so getting the mix right can be difficult. If you really want a lighter color that is between two shades it can be done, but I generally will buy a sample of the lighter shade and the darker shade and find out if they are both mixed with the same base. If they are in the same base you can play with the colors and mix the lighter and darker colors in different ratios until you find the perfect shade. I use syringes to measure the amounts so I am sure that the ratio is measured accurately. Then I do my own math to determine the amount of each colorant used in the samples and provide the paint store with the base color and the formula that has resulted in the color I want. If they are in two different bases you can do the same thing, but you will need to buy the appropriate amount of each paint and mix them together in a 5 gallon pail with an paint mixer drill attachment or ask the paint store to mix using their mixer. The same applies if you are trying to get a color that is just a tinge more green or yellow or blue than another. Tools like easyrgb have made a lot of this unnecessary - I can find a similar color in another brand and have it matched by my BM or SW paint store.

I use flat on ceilings, eggshell on walls and semi gloss on trim and doors.

@Jay W There are no rules that can't be broken, and I have even used 2 neutrals with two different undertones in one open concept space. The living room had green carpet and the sofa was a patterned fall color sofa with orange/gold and greens. We picked an orange undertone for the walls in that area and used a green undertone in the kitchen and dining room that had hard flooring with an orange undertone and woodwork that has some orange tones. The green was a stronger, more obvious green undertone, not just a tinge of green (The color was named Moss). Both rooms had both undertones in similar amounts and strong colors, the neutrals were more less just flip flopped. This is a bit more difficult to pull off than using just one undertone or similar undertones. If the majority is orange/pink/purple undertones and you have just a single room with a greige or green beige the greige or green beige may feel dingy in comparison. If you have it in a bedroom, but have bedding and an area carpet that pulls in some of the Nomadic desert color and the White Duck colors it may provide a calming restful feeling, which is appropriate for a bedroom. I would also try to get a piece of art for the hallway that has both colors so the brain recognizes the connection between the rooms.