You may use the isGroupLayer property to identify group layers to ignore them. To ignore feature dataset layers you could use the dataSource property to evaluate the layer path and ignore those that have an extra level deep being the feature dataset layers inside the database.
mxd = arcpy.mapping.MapDocument("G:\\zn\\NM\\GGB\\Applicaties-Admin\\Kerngis\\...
in the raster calculator, the symbol "^" is a boolean operator, not the power function.
For the square values, you should either use the Power() function or the equivalent operator **
"prec_1" ** 2
In addition, make sure that you work in float, before your percentages could be rounded to zero before you multiply by 100.