How to do the timing constraint?

I have used this methodology in the past (that's why I know it) and it does work.

And of course, if you apply this methodology to your example design, you're going to get exactly the same result. The minimum solution the fitter gave you, with a 15 ps tCO, will also meet a 0/7.8125 min/max output delay constraint by a mile.

As you may have noticed, I really dislike not having solid time constrains. Independently of weather a unconstrained design meets timing requirements by a mile or I have to manually place bits of logic to meet timing, I _always_ try to have solid constraints at my starting point.