Ratio to subtitute mesquite liquid smoke for mesquite smoke powder?
Because of the strength of liquid smoke, it's hard to provide a substitution ratio without knowledge of the full recipe you are using.

As you mentioned, liquid smoke is pretty strong stuff. Smoke powder seems to be made from liquid smoke, thus the powdered smoke may be more concentrated than the liquid smoke. 

Many comments on smoke powders on Amazon mention that a little goes a long way. According to this official description this powder is "tenfold-strength":


  Mesquite smoke powder is made from real mesquite smoke, which is spray-dried onto a neutral maltodextrin base. Shake a little of this tenfold-strength mesquite powder into barbecue sauce, baked beans, or homemade potato chips for an addictively sweet and smoky flavor. 
  Ingredients:Corn Flour, Natural Mesquite Flavor. Gluten free.
  
  – thespicehouse.com/mesquite-smoke-powder


Another consideration when using liquid smoke instead of powdered smoke is if the powder part (maltodextrin) plays any part in the formation of the seitan. This description of another smoke powder mentions that the powder is "highly hygroscopic" (it absorbs water) thus if substituting liquid smoke you may want to also add some maltodextrin or perhaps a little cornstarch.


  This smoke powder is to be used as a dry rub as it is highly hygroscopic in water, meaning it balls up when combined with liquid and does not mix well. When using this as dry rub, I recommend that it's used at a rate of 1oz-5oz per 100 lbs of meat (It really doesn¡¦t take much to get a very good smoky flavor). I also recommend that you use gloves when applying this or you will smell like hickory smoke for a while :)º Alternatively, you can mix the smoke powder with a small amount of vegetable oil, creating a paste, and apply it to your protein that way. A tiny bit of vinegar can also help break up the smoke powder and won¡¦t impact the flavor at all. After you have applied the rub or paste to your protein, you should let your protein rest so that all of the smoke flavor permeates the meat. I always use the smoke powder as a dry rub and have transferred it into a salt shaker for easy application. 
  
  – comment from The Colorado Spice Team on Amazon


If you are open to other liquid smoke substitutes:


Smoked Paprika Powder
Chipotle Powder
Smoke Tea


If you wish to use liquid smoke as your substitute, I would suggest that you look at a few other seitan recipes which use liquid smoke and adjust based upon your recipe’s yield. 

Also take a look at this guide on How to Make Perfect Seitan