I think "nort" and "go" have nailed it. Get a weight that is good and as long as you don't start carrying another elephant in the front compartment any changes from trip to trip will be negligible. Adding 200# to your TT should affect your tongue by ~26#. If that's going to put you over weight on the tongue you should be shopping for a bigger truck, smaller TT or leave the elephant at home.
"Night" has a good suggestion as well, maybe hitting the scale every spring (or so) is just good mojo.
I also think the CAT Scale accuracy is about +-10# anyway.
All this said, guess what?
The CAT Scale is your friend.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks