Lots of sites(including this one) have guide locating tables for rods of varying lengths. The manufacturer or seller of the blank can also supply good info usually.
If you use a table or calculator, remember that it may not give you the *optimal* guide spacing for your blank. What someone recommended to me was to use a table and layout the guides, attaching them temporarily with tape/elastic/string. Then string the rod and check to see how well the line follows the curve of the rod when loaded(the rod...not the builder). Adjust as needed.