I use it to taste, but I think 1" of paste would be similar to one stalk (perhaps a teaspoon or two), but it will never be exactly the same, as the flavor of fresh has nuances that are lost in the tube. However, the tube is the best substitute, and I use it frequently. I often add a bit of Yuzu juice to bolster the flavor. You can buy yuzu juice in bottles, but I have plenty of yuzu on my tree, and the trees are easy to grow - they are hardy to zone 8 and possibly zone 7. I think you can also grow lemon grass and then freeze it until ready to use. I use yuzu zest in addition to the juice as a substitute for lemon grass.

Lemon grass stalks vary quite a bit in flavor when fresh, depending on how large they are and how old they are, and so "one stalk" is not a very exact measurement.

Well, you can get fresh through the mail, but the price is ridiculous. When you say tube, do you mean it's a paste? I've never seen that. Does the maker's website say? Have you tasted it? A lot of powdered lemongrass is meant for tea, and isn't as flavorful. Chopped and dried or freeze dried is more common. I'd put in a handful of dried and check for taste. If it's a paste in a tube and has a strong flavor, maybe start with a tablespoon? That's just a guess, however. Think of this as an opportunity to make the recipe your own by adding it to taste rather than to measure. :)

Good luck!

I assume you're referring to Garden Gourmet. I use it too. To me it's a little sweet, so I think what Lars does with the Yuzu juice might help a bit. It's definitely convenient. The lemon grass stalks sold in grocery stores can often be so dried out that they're impossible to smash, mince, or whatever. So the paste in tubes is really helpful.

Garden Gourmet

I tend to use lemongrass in the tube over fresh most of the time.

Of course it depends on how big a meal you are cooking but I tend to use about a third of a tube per meal for the two of us.