I think it also depends on the quality of the Ice - if it is clear solid ice you can probably be safe at 6 but if is it cloudy with mulitple layers of snow/sleet or bad ice then maybe not.
There is no exact science to what is safe, especially when you can have 6 inches in one spot and 3 in another depending on conditions.
The link below has a good review of what the DNR recomends and explains how they go to the numbers. Scroll down it a bit and you can see a table ice thickness and weight.
Another factor to take into account is the weight distribution. If you are pulling the gear and sled you may not need to count that weight as it is distributed over a larger area.http://lakeice.squarespace.com/bearing-strength/
Be careful not matter what and good luck.