Here is a sub tool that I made so you can have a grid inside of Zbush. I made a centimeter and meter version.
This Ztool is very easy to use with preexisting models when used in conjunction with subtool master. (Just use the Multi Append button and load either your centimeter or meter grid. This will auto load all the pieces of the grid to your preexisting model.)
This grid has helped me place the feet of my characters on the ground, I hope this helps you out as well.