If it were me, I would hold out for 9 or 10 GOOD inches. On this site somewhere is a graph of thickness vs weight An atv or snowmobile is good at five inches or better. Most articles recommend to actually use a tape measure in stead of guessing the ice thickness. I am waiting for six inches down on Flaming Gorge as pulling my sled half a mile is getting me in too good of condition.
Hunt up google corp of engineers site for ice loading as well.
