How much contact in too much?

[umpire99]

How much contact does there need to be between a baserunner and a fielder who is attempting to field the ball before interference is called?

I had on with some contact in a FED game today. When the coach asked if there was contact I told him that there was not enough contact for me to call interference. When he then asked how much contact there needs to be I realized that maybe any contact at all is cause for interference so I changed my answer to "In my judgement, there was no contact." I knew that using the phrase "in my judgement" would stop the discussion there.