How much yarn is enough yarn?

So I have this pattern, a very cute pattern that asks for 219 yards of yarn, well 94gm of yarn that is 234yard/100 grams. Which if you do the math (very very loose math that it is), would take 219 yards of yarn, of that specific weight.

I have 2 balls of gorgeous, absolutely gorgeous navy blue wool/silk yarn that weighs in at 171 yards/25gm... If I were to double it up, I should be able to match guage (I think I may be dreaming HUGE fantasies here, but let me dream on for a bit), and then I might (if I knit fast enough) be able to complete the pattern right? It SHOULD be enough yarn... I think... Maybe... *sigh*

I would absolutely hate getting to the end of the yarn and have less than an inch to go at the top of the thing!, that would absolutely suck... so maybe I need to pick another yarn out of my stash to work with...

So this other yarn, that I have been wanting to cast on with (100% baby alpaca, how yummy is that), is 50gm/unknown yardage. it looks pretty thin. I do have 2 balls of it, so if I double it up, it *looks* like I would have enough yarn, but I donno... Would it be thick enough? Perhaps I need to go do some WPI measurements with these yarns, maybe that would help me decide (I think not).

Or if I were really proactive I could do a swatch (not that the pattern has anything other than 22s/4", which doesn't help me much cause I STILL DON'T KNOW how much yarn it will take me to knit the thing.

Perhaps I should just cast on and hope it will all work out. If I believe hard enough, it will come true, Right? Or perhaps I should just wait a bit and see if I have something else in my stash that would be more appropriate for the project... but this wool/silk is boootiful, and the baby alpaca is sooo soft and squishy.

Hrm... Decisions... Decisions...