Removal of old flowers prolongs the blooming period, reduces self seeding which leads to volunteer plants, and promotes flowering on side shoots. Cut off spent flower spikes just below the lowest floret. To be effective, removal must be done soon after the flowers fade.
Generally allow about 2-in. from the back of the tool to the wall, but be sure to leave space for any sawdust collection equipment you plan to install, including the ductwork and any attachments to it or to the tool.