…safe to do? How much should be left as a minimum in the frame?

are you sawing it to save weight or because it’s too long?

in most cases it’s the frame that determines minimum insert not the seatpost – ideally you want a couple of inches of seatpost below the top tube/seatstay junction on the seat tube.

other option is to work out what length seatpost you want and just buy one that long, I guess.

Saving weight. No point spending and throwing away what 5 mins with a hacksaw and some glasspaper can achieve…

Just wanted to make sure it wasnt dangerous.

it won’t be as good as keepign as much seatpost in the frame as you can…

you’ll save about 30 grammes – you’d probably save more having a good sneeze before you ride.