Sy, there is no hard number here, just things to consider. Like wheels don't always stay true on a ride. Stuff happens. If the wheel goes out of true (breaks a spoke, you crash, ...), are you willing to risk scraping off the faint and ever abrading the carbon fiber a bit to ride home or call a spouse or other for a ride to protect the fork? If you have steel, it's just paint, but not many ride steel forks any more. The risk does go up of having an object jam between the tire and fork. Not likely but still ...
These are all judgement/degree of risk calls.
There is a remedy, but it is a step back in time. Go to a frame maker and have him build you a steel fork with appropriate steerer. Steel is, spacewise, a far more efficient material. For the same distance of fork crown top to hub, you would have a wide choice of fork crowns that were wider and higher over the tire. Brace yourself if you think you want to go this route. The fork will cost you $4-700 and add an easy pound to the bike weight.
An incident of relevance here. Early on in the life of my moniker, that 1979 Peter Mooney, I was in a town line sprint. A rider stuck his quick release into my spokes. I leaned into his wheel to be above to separate from him and not crash. I rode the bike to a standstill with 8 consecutive spokes on that side broken. There was a 1/2" patch of bare steel on the inside of the brand new fork. This on a fork that had a mile of tire clearance. Had the clearance been tight, I highly doubt I could have saved myself from crashing. As it was, had the fork been carbon fiber, it would have been compromised severely. As I said above, stuff happens. Yes this was "competition", but sticks and dogs also get into bike spokes.
Ben