I have found from experience (depending on media quailty) that it is preferable to avoid recording to the maximum limit of a disk (e.g. don't go beyond 4MB as a maximum - it is often the outer edge of a disk where problems arise - even good quality media suffers this from time to time).
Typically, I try and avoid using the last 20% or so of the disk, and I rarely have problems as a result. I accept I am being conservative, but its so blinking annoying if a recording fails or freezes etc near the end!
Thus if I have a 0 to 1 hr 40 min prog, I nornally use 2 hr mode.
Thus if I have a 1 hr 40 min to 2hr 40 min prog, I normally use 3 hr mode.
Beyond that - 4 hour mode. Forget 6 hour mode - it is bad.