sorry if i came across rude. the best thing to do is keep a 1:1 ratio
if you want to cut 1in off, push the nut down 1in.
this may not be necessary, but it will ensure you dont mess up. if you can, find a friend with the "slide hammer" made for doing this job. its important to keep the nut straight and with the correct tools its easy and idiot proof.
if you cant get a hold of a star nut tool, you can use a wood dowel, tube, or screwdriver handle that fits in the steerer tube with a little bit of play (so it doesn't get stuck) then gently tap it down with a hammer.
Screw the top cap's screw into the star nut about halfway, making sure there's enough bolt head sticking out of the top of the steerer to hit, and then tap it gently and squarely with a hammer. Take the bolt back out once the star nut has gone down the proper amount, cut the tube, file the edges, and re-install.