Well, the default you steal from other 52840 espruino boards is probably ok, numbers are computed for espruino bootloader size/location. So if you cleared adafruit bootloader or won't use any bootloader (which is also possible) they are probably OK, see explanations in comments here https://github.com/espruino/Espruino/blob/master/boards/NRF52840DK.py#L62
There are 256 4KB pages in 1MB
EDIT: however the 31 looks fishy there in 'Softdevice uses 31 pages of flash', that would be 0x1f000 which is for S132 3.x and not right for S140 6.x. Good table is here so for S140 6.x there should be 0x26000/4096=38 instead of 31
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Well, the default you steal from other 52840 espruino boards is probably ok, numbers are computed for espruino bootloader size/location. So if you cleared adafruit bootloader or won't use any bootloader (which is also possible) they are probably OK, see explanations in comments here https://github.com/espruino/Espruino/blob/master/boards/NRF52840DK.py#L62
There are 256 4KB pages in 1MB
EDIT: however the 31 looks fishy there in 'Softdevice uses 31 pages of flash', that would be 0x1f000 which is for S132 3.x and not right for S140 6.x. Good table is here so for S140 6.x there should be 0x26000/4096=38 instead of 31