It's 1.75 times the xp to level, not 1.5. A second TR will be an additional 1.75 on top of that.
For example, it takes 190,000 xp to go from 19 to 20. A TR needs 332,500xp instead. A TR2 needs 581,875.
In the earlier levels, it shouldn't matter because there is so much more stuff to run. Later on, we're going to be pressed for xp, and the TR's may have to bank stuff. We WILL need to run some raids at level, VoN for example can net upwards of 30k in a single run. That is too much to pass up. Up to Gianthold Level, we should be fine; but past Gianthold/Orchard there's simply not that much. Granted, there (hopefully) may be be more, and it will be a moot issue once we get there.
However, that means we really don't need to bank on the penalties and bonuses balancing out. Running stuff with no one more than 1 level above the quest level is fine. Some stuff we'll want to hold off on until later, like IQ, as of right now there's only IQ and Shavarath at the 18-20 range, and of course the Hound of Xoriat and Vision of Destruction raids.
I'm working OT for the foreseeable future, unless I find another job first or win the lotto. So weekday runs will be rare for me for the time being.
I also have a Bard and a Sorc down at 7 and 9, respectively.