Tire pressure for road slicks on a mountain bike

Hi, I'm working on getting my s together so to speak for my first triathlon.  For the biking portion, I decided to to put road slicks on my mountain bike.  The question I have is, any feeling on what tire pressure I should use for the road tires?  I'm about 195 pounds.  I've read that mountain bike tires should be at about 40 psi and road bikes at about 100 psi, so I'm guessing I'd be somewhere in between?  I filled them up to about 80 psi, and just by feel it doesn't seem like enough pressure.  My bike pump has a gauge and it seems like my mountain bike tires were well above 40 psi though, so maybe that's what's confusing me.  Any thoughts?