The general rule of thumb when it comes to teardrops is 1/3 of the
body length from the rearmost part of the body. On a 10' body (120"), the 40"-42" measurement is pretty close, but in your case, 38-40" would be more like it.
Of course, a
lot depends on how much and where you locate your cabinetry (as well as any appliances you plan to install), if you have a front closet/storage-area, and/or if you plan to install a tongue-box (or anything else) on the tongue. Some of us have been doing this long enough that we can look at all of that and say the axle centerline needs to go "there" ... others need a formula to follow ... but it
usually all works out in the end ...

...!
Good Luck! In the meantime ...
CHEERS!
Grant
Celebrating
Retirement after over
32 Years of Building, Promoting, Supporting, Supplying, Living the Lifestyle, and Loving
Teardrop Trailers!"Life Moves a Little Slower When You're On Teardrop Time"The nature of Life, itself, is change ...
"Those who matter, don't mind, and those who mind, don't matter." 