File-fit means exactly that, rings are manufactured with minimal end gap which requires filing to obtain the desired end gap. On the other hand
pre-gapped rings don't require fitting, however you should check end-gap by installing each ring in its bore. Square it up about an inch down the bore and check gap with your feeler gauges. This is critical as I've seen boxes labeled for a particular bore size that contained rings sized for another.
How much end gap a ring should have depends on its material, bore size and application--street or racing and normally aspirated, turbocharged/supercharged or nitrous. Also, the type of fuel used has to be considered, i.e., gasoline, alcohol or whatever. Cooler operating alcohol doesn't require as much end gap. For normal street operation with gasoline and ductile top rings, 0.004 in. per inch of bore size will work. Increase this to 0.005 in. per inch of bore size for racing applications. Second ring-end gap should be about 0.0055 in. per inch of bore size.
Oil-ring rails can be all the same at 0.015 in. per inch of bore size.
For a 4.00-in. bore, ring gaps should be 0.018--0.020 in. top; 0.022--0.024 in. second and 0.016--0.020 in.
oil rail for road racing. With turbocharging/supercharging gap rings to 0.024--0.028 in. top; 0.024--0.026 in, second and 0.016--0.020 in.
oil. For nitrous applications, gap the rings 0.028--0.030 in. top; 0.028--0.030 in. second and 0.016--0.020 in. oil.
Keep in mind that when gapping rings, a larger end gap is much safer that not enough. If the ring ends were to butt because of insufficient gap, serious ring and bore damage could result. Testing with very large gaps has shown that the increse in blowby is virtually imperceptible. On the other hand, there are those who swear by Total Seal's gapless and Childs & Albert's Zero Gap rings used in the second grooves.