Same as the angle of the chain. It is recommended you use your round file that accommodates the size of the tooth of the chain. count the cuts or motions (each tooth should be filed the same amount) if you cut the first tooth three times cut the balance three times. keep the exact angle of the tooth and do not deviate or the chain will not cut as well. if you over file and change the angle for the chain you are filing you may need to have a professional fix it or buy a new chain. the flat file is simply a light stroke to remove misc fragments off the top of the tooth or give it a sharper edge after the round filing is complete. when using a round file it is easiest to purchase a small holder so each stroke is stable and straight. good luck.