it depends on what kind of shrooms they are. if they are cubensis, then take half an 1/8th. heres what the top of a cubensis cap looks like.http://www.erowid.org/plants/show_im...cubensis19.jpg
although my last trip i ate a little less than half an 1/8th, and tripped just as hard if not harder than when i did a whole 1/8th.