Its not easy to weigh a mast so I have only an estimate of the weight of mine - 25 kg at least, and maybe getting towards 30kg. It ended up much heavier than I wanted, and much much heavier than the mast plus rigging it replaced. The boat is only 1700 lb and has no ballasted keel, only a centre board. (There may be a little bit of ballast in the stub.) Anyway, for what that is worth, surprisingly to me, the mast does not seem too heavy, she stands up quite well in a breeze - though it is about the limit for me these days, for stepping. [edit: I can do it though - I have a good tabernacle. For a younger person 30kg would be "up there", but OK I think provided it is being raised in a tabernacle.]
If your 28' lamp post weighs 35 kg then by simple proportion cutting it down to 24' will bring it to something like (24/28)*35 = 30 kg. (Maybe a little under that, since you will be cutting off the heavy end.)
James G is carrying a 37kg mast on his 19' (1200 lb) River Rat and says it is too heavy, though he is sailing with it.
I agree with Arne, but would go further: don't cut the mast at all until you have made the sail and hoisted it high enough to be out of the lifts and lazy jacks. Then you can take the mast down and cut off what you don't need, from the bottom end allowing a little extra, as Arne suggests. Don't cut it too short, is the lesson I learned.