I put a level indicator right on the front of the trailer just under the bunk door so I can see it at the hitch. That's the one we worry about when backing in. If it's off, then leveling blocks are called for. I put the other one on the street side at the front. Once I've disconnected the TV, I can crank the front jack up/down until it's level and the indicator is right there. I agree with getting within 1/2 a bubble, that's good enough.
I think using any kind of level indicator inside the trailer is terribly inconvenient, as mentioned above.
You do need to make sure the trailer is absolutely level before you install the level indicators, or you'll always be off.
1988 Coleman Sequoia - popup (1987-2007)
2012 Roo19 - hybid (2012-2015)
2016 Mini Lite 2503S - tt (2015 - ???)
2011 Traverse LT, 3.6L, FWD
2009 Silverado Ext Cab, 5.3L, 4x4, 3.73
2016 Silverado 2500HD Dbl Cab, 6.0L 4x4, 4.10