DFU for nRF9160 with max possible image size

Hello, I develop the dfu for my application. I started testing the dfu process with maximum image size that I can build and I need some help here.

Here is my pm_static.yml:

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# flash primary:
mcuboot:
address: 0x0
end_address: 0x10000
placement:
before:
- mcuboot_primary
region: flash_primary
size: 0x10000
mcuboot_primary:
address: 0x10000
end_address: 0xe8000
orig_span: &id001
- mcuboot_pad
- app
- tfm
region: flash_primary
size: 0xd8000
span: *id001
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

I had to limit the imgtool max slot size to 4kB less than mcuboot secondary slot size with the following configuration:

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CONFIG_MCUBOOT_EXTRA_IMGTOOL_ARGS="--slot-size 0xD6000"
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Otherwise, the mcuboot says that the image in secondary slot is not valid (too big). 

With such configuration, I can perform dfu for max possible image size that I can build and everything seems to work fine, but the mcuboot prints the following warning:

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Non-optimal sector distribution, slot0 has 214 usable sectors (216 assigned) but slot1 has 215 assigned
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

That suggests the mcuboot secondary slot should be 1 sector lower (4kB) but I don't understand why. If I lower the mcuboot secondary slot by 1 sector, the warning disappears, but I also have to lower the imgtool slot size argument to 0xD5000, to make the dfu pass (which seems to be wasting 1 sector).

So my questions are:
1. Is my pm_static.yml configured okay? Is it the correct way to reserve 1 sector (4kB) for swap using move algorithm?
2. Why mcuboot warns me about non-optimal sector distribution?
3. Is it necessary to limit the imgtool output to 1 memory sector less than the mcuboot secondary slot size?

nRF9160, NCS 2.9.0