I have this problem a lot in my projects. I tend to run a hybrid of what you and Larry are talking about, because Larry is right backwards scheduling can (is) a nightmare. I tend to get all my task in place and as Larry mentioned create a good set of milestones. Then I try to fill in any times I know or can estimate 90% or better. Set your go live date as a must finish by and then link all your task as you want to. If you know the majority of your times this will almost set the timing for the ones you do not know. I use it a lot in motivating my engineers with tool runoffs. Customer SOP is set tooling complete date is known, whats left in between there is tool runoff and PPAp timing. When you set it this way once a task exceeds your end date Project will turn it read and let you know you have issues. Then you start Program manager wheeling and dealing to make up the time.
Hope this helps. I am sure Larry will agree this is one of those things where there is 10 ways to skin a cat, you just have to try a cuple and see what works for you.