Cutting off an amount of steerer equal to the spacers you are removing is faulty logic because it makes assumptions about how good the steerer gap (that 2-3mm distance) is now. When you have spacers above the stem, that gap number could be anything and it doesn't matter because the stem is making full contact with the steerer. But if you're not going to have spacers above the stem anymore, it must be correct.
A good method is to put the stem to the level you want it, leave it clamped on, remove everything above it, then mark the steerer with a fine-tip marker or scribe at the top of it. Then remove the fork and make another mark 2mm below the first mark. That is your cut line. If your plan for cutting favors having the line encircle the steerer, just temporarily clamp the stem to match the cut mark and mark around that way.
It's very easy for things to go wrong cutting a steerer freehand, especially a carbon or aluminum one since the material is soft and things can happen faster. Even a basic saw guide made out of a scrap of angle iron is much better, and since you have a hacksaw you have the means to make one.
A 32tpi blade can be fine on carbon. Use a lot of water to lubricate it and also keep the dust out of the air. I like to do the trick of turning the steerer 90 degrees or so in the guide when the cut is almost complete to help prevent tearing as you go through the wall.