I think the goal for most of us is to be good at percussion and just use pressure to clean up a point,but you need to learn pressure and glass bottles are a cheap start ,and you will ruin alot of material at the start ,I would not invest in precut preforms ,they will cost you ,slabs are a little cheaper ,or just do it the natural way just get a load of flakes or spalls and try to learn it all at once,percussion and pressure ,Bill Skinner was nice enough to me to send me a box of rocks to start with for free and I came up with alot of glass windows and bottles ,I am coming along ,that hornstone is made for percussion it knapps beautiful.