In January I knew from Christmases past, who I will be giving...

In January I knew from Christmases past, who I will be giving presents to, and about how much I would be spending.

If anyone asked I could not tell them what I would get or for whom, but I knew how much money I needed to put aside in my Christmas club.

Now it is getting closer to execution I know I am getting my wife a bracelet she admires. So now that I know exactly the gift, I can conduct a survey of different vendors to see where I can get it, and how much it will cost. With that information I can then make more precise estimates of who else gets what and at what price, all within (give or take) my January plan.

I also know I need wrapping paper, but I am not buying it in June when I still don't know what presents I will get, how big they are, and exactly how much paper I will need. Wrapping paper purchase is not on the critical path and I will probably leave it until Christmas Eve to make those decisions and still adequately populate the tree.

The analogy is ok as far as it goes (not totally accurate because it has a fixed target date). But I'm sure you get my point; in any large project planning exercise you can use past experience to extrapolate ballpark estimates of overall project costs (how much you need to save or raise). Some activities will cost more, some less - and in case the whole thing goes pear shaped I allow a contingency.

I then manage the project as I go, refining estimates and making appropriate decisions about who gets what to bring it home within budget.

And yes I can tell anyone in January how much I will spend on presents next Christmas, but cannot (yet) give detail on who gets what, that will be refined as I go.