The rating system is not intended to be a personal handicap system but is designed to rate the potential of the boat (i.e. if Randy Smyth was sailing it). Your, my, or Joe Blow's skill should have no bearing on the rating. At the end of the day, lets find a way to do what most feel is fair. I think the number 67.9 or 67.6 as you mentioned is a fair starting point. If it looks like you're unfairly clobering guys with that rating or there is a lot of discourse, then let's talk about it again.

On the rating / time calculation; you use the handicap number with the following forumla:

CT (Corrected Time)
ET (Elapsed Time)
HC (Handicap)

CT = (ET/HC) * 100

For a one hour race with an F18 rating of 62.4 the corrected time will be (60/62.4)*100 = 96.15 minutes

For the Hobie18Thang at 67.6 the corrected time would be (60/67.6)*100 = 88.76 minutes

The difference is 7.4 minutes. Understanding that the numbers work out a little differently if the Hobie18Thang is finishing in 68 minutes, approximating an identical 60 minute finish gives you some basis for comparison. The F18 needs to better the 18Thang by a time around 7.4 minutes in a one hour race in order to beat it on corrected time.