Also keep in mind that there is a huge difference between single turn in leves in the same level range. The ones where the quest starter and client are both in the city give the least amount of base exp. Ones where the quest starter and client are both in a outpost type area (Costa, Coerthas, etc) tend to give better base exp. Ones where the quest starter is in the city but the client is at an outpost give the best exp, since it requires having to travel between quest giver and client.
As far as your level 45 Carpentry levequest example, I'm guessing some of your information is pre-patch. The same patch where they lowered HQ bonus from 200% to 100%, they also buffed the base exp for many single turn in levequests. The highest city-to-city leve is
Clogs of War, which gives base exp of 71,040. The highest outpost-to-outpost one is
Bow Down to Magic, which gives base exp of 72,932. The highest city-to-outpost one is
Trident and Error, which gives base exp of 103,656.
Basically, you can ignore HQ bonus as a factor when comparing leves since it's exactly the same boost to exp reward regardless of whether it's a triple turn in or single turn in leve. The only time HQ bonus might be a factor when deciding is if one of the levequests requires turning in 40 durability recipes, which tend to be harder to HQ than the 60/70/80 durability recipes at the same level.
So to compare triple turn to single turn, multiply the base exp reward for the triple turn in and compare it to the best single turn in. In this case,
Incant Now, Think Later has a base exp reward of 33,921. So doing all 3 turn ins on a single leve gets 101,763 exp before HQ bonus. Compare that to the outpost-to-city leve, which has base exp of 103,656.
In this example, the single turn is is actually superior to the triple turn in, even as far as efficient use of leve allowances. If you have a security token, you could even set your free teleport point to either Gridania or Mor Dhona, and set the other destination to a favorite teleport destination. You could even set the favorite destination to a home point, and you get a free trip back there every 15 minutes with Return.