It's your dive logbook you can do as told above - as you like. Or, for now, you can follow standard training protocol and follow the above instructions from the RDP instructions. Again, your call.
i understand the difference between the two, looking at my bottom time is just something i don't normally do or remember if i do and it is easy to pull the time from the computer at a later time.
The sites i dive tend to have some vertical profile to them so the times i have gone back and checked the PG and NDL limit from the table based on the bottom time the computer let me do i have always been well outside of the limits so i cant see it being a back up in case my computer fail (as in to transition to tables for future dives, if a computer fails on a dive that dive is over). I don't have any experience manually calculating multi level dives
but i'm getting off topic, i've just wondered why my PADI logbook only had room for bottom time not dive time or both when the majority of diving in off computers these days, might just be the case that not much thought goes into it