The Rose factor does not apply to all cakes, just to the downy yellow, the white velvet and the all-american chocolate. You can use the charts to “guesstimate” for other cakes, but then it would be a starting point for test cakes, not a tested, sure-fire, result.
Are you trying to make two 8” x 2” layers of downy yellow? According to the chart on p. 455 of TCB, one 8x2 pan holds 7 cups, and one 9 x 1.5 inch pan holds about 6.5 cups. So the recipe on p.39 would probably make two 8 x 2” layers, though the batter might be a tad short. You will have a bit less batter and your layers will not be as high as if you use the Rose factor system.
For the Rose factor, the chart on p. 490 does indicate that 9” cakes are a level 2 for baking powder while 8” cakes are level 1. It is not a big difference, if you leave the baking powder unchanged (i.e., ignore the Rose factor adjustment), be sure to use cake strips and be sure your oven isn’t running hot. Your layers may bake up with a rounded top rather than flat.
If you want to adjust the baking powder so as to get a flat top to the layers, learn to use the charts.
From the chart on p. 490, you can see that the 8” layer is a level 1 with a Rose factor of 3.5. Next, go to the first chart on p. 492, and multiply the base recipe by the Rose factor of 3.5 to get the correct amouunt of batter (for two 8x2” layers). Next, go to the second chart on p. 492, “Baking Powder Amounts for Yellow/White cakes.” It indicates that you need 1.5 tsp baking powder for a level 1 cake, multiplied by the Rose factor of 3.5. So, 1.5 x 3.5 =5.25 tsp baking powder. Using this system, you will have a little more batter, higher layers, and an even, flat shape.