Filing straight across is 0 Degree filing angle and will put a 90 degree angle on your side cutter. Filing at a 25 degree filing angle puts a 65 degree angle on your side cutter (approximately because the side cutter is itself angled about 6 degrees 90 - 25 = 65 ) . A 30 degree filing angle will give a more aggressive but also more fragile cutting edge to the side cutter and a more projecting hook to the top cutter. The sharper the filing angle the more aggressive but if you hit dirt the chain gets damaged worse and needs more cutter filed off to resharpen.