I measure the stones at the side,to gauge how deep,pick the best ones,if you can’t my guide is 50mm core drill 30 deep,then 25/30 twist drill,sds,for another 150,that’s with my usual 30 square post,with a stub of 16 mm welded,which I cut slits in ,then open up with a chisel,on a landing,where people congregate and lean a lot,then I’ll go in 250 approximately depending on the stone size and quality.