You need to move the hub axle approximately 1/8" in the dropout for each tooth change, so in your example with 18/20T freewheels it would be 1/4". You will need about 1/4" clearance to the inside of the dropouts to get enough slack in the chain to remove it from the chainring so you can remove the rear wheel. So, you can actually accomodate a significant change in ratio with at typical track dropout.
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