Not sure if its the right idea to press LNAV while on HDG Sel getting radar vectored. You would not want to get things messy in terminal area during high work load.
For VNAV calculations while not in LNAV is tricky. I have not seen any recommended procedure for this as well which is surprising esp from manufacturers because 737s just dont descend. So what are the hard and fast rules? Greatly appreciate if someone can list one. Mental calculation is there that multiplication of three but it can be sloppy and misguided because we are not getting the exact track miles all the time.