adjusting mission timer mid-mission?

Is it possible to change the mission timer in the middle of a mission?

I like being able to set the mission timer to what I think the flight time will be (given the conditions) in order to estimate the number of batteries I'll use, then set that mission timer a bit higher prior to starting the mission such that I have a fail safe beyond the "low battery return to home" feature. That is, my expectation is that the return to home will be triggered by low battery, but it will certainly be triggered by the flight timer running out.

Sometimes I guess pretty wrong about how conditions might affect my flight time (especially the terrain that I encounter and elevation I'm at). It might be nice to adjust the flight timer up or down during battery switches if, for instance, I realize I can probably get another minute of flight more than what I've set the timer to.

Perhaps the preferred way to use the software would be to set the flight timer to what I think will actually be flown to estimate battery usage, and then set it to "battery limited" for actual flying (instead of trying to guess what the true flight time will be and then adding a minute). That is, maybe the extra precaution of setting a flight timer near what the true flight time will be doesn't really act as a precaution anyway.

What do you think would be the best way to approach this?