Already bought a 22mm for steel and looking for others,used OK,must be for use with steel,otherwise a reamer could help it's an alloy post and I'm using a standard drill with adapter for larger drill bits
When I did the same to my Kona, basically milling the seatpost downwards VERY carefully, I used holesaws from here
You'll need the suitable arbor as well.
Damn them holesaws are expensive i think i'll stick to collecting drill bits,sometimes the best bet is to drill out to 22mm then use a bmx post,using caustic soda will take a very long time due to depth of post