CI54
I think the whole idea is to have a backup method in your head which confirms what the FMC is telling you, or tells you when it has screwed up the VNAV profile due to several updates to the LNAV track.
Regards
PP
ps simplest method has to be 3xheight to lose (in thousands) plus 1nm for every 10kts airspeed you have to lose down to your 'centre fix speed'. Works 99/100. Just put a fix in on the field whilst the other pilot is madly updating LNAV track every time your heading changes and guesstimate the track miles to the field. You are only trying to get in the ballpark to end up at about 15nm to run with the speed and height under control.