Say a high magnification scope in a LT104. How are y'all centering the cross hairs in the mount?

Wheeler scope leveling kit or a plumb bob.

Arisaka engineering  scope leveling tool

Segway level

This has made my life much easier. The only problem is when I get a set of low scope rings or a mount that won't allow it to fit.

To the OP, this kit works great on the LT104.

I level with a feeler gauge. That generally works well for me.  Depending on how your rifle fits you, you may need to level the scope when you are in your shooting position.

I don't necessarily center the scope in the rings.  I position the mount where I want it on the rifle and then get behind the rifle and slide the scope back and forth until eye relief is correct.  That's more important than being centered in the rings.

I assume you meant leveling vs centering. I've always used a plumb bob and string. I use a bubble level and/or lately my iPhone compass level feature to get the rifle perfectly level then match the vertical cross hairs to the plumb bob string. 

+1. Getting proper eye relief in my natural shooting position is better than having it perfectly centered.

This is a great little tool.

Badger DeadLevel and a plumb line.

My method may not be the best but it works for me to get my crosshairs level to the rifle, level to the ground. I use 2 inexpensive tiny "line" levels from Lowes for $4.

I first establish generally where I want the rings to sit on the upper, and generally where I want the scope to rest in the rings (front/aft) for my specific eye relief. Establish the ring location and fully install the rings to the upper and get that finished off and torqued down. Then play around with the scope loosely in the rigs and move it front/aft, under all the magnification levels to finalize where you want the scope to rest for your eye relief. Consider marking the scope tube with tape somewhere to temporarily mark the location (front/aft), just in case the scope moves while you put the rifle in a stationary position like a mag vise, etc.

Now level the rifle. Put one level on the upper just forward of the charging handle and get the rifle level front to back. Then reposition level 90 degrees and level the rifle right to left. Leave that level there and put the other level on a flat spot on top of the scope. Make sure scope hasn't moved and its in proper forward/aft location. Rotate scope accordingly to make it level. Now both levels should be level to the ground. Tighten up the rings and you're done. You should now have scope crosshairs that are for all practical purposes level to the rifle, which is level to the ground.

I use the kit from Wheeler.

I use a round bubble level and a small line level. The rifle goes in a led sled. One level goes on the scope. The other goes on the receiver, if possible. When both levels are level, the rings get torqued making sure both levels stay level.

This.

If you meant leveling, then level your rifle with bubbles......while the scope is snug enough to not slip, but not so tight that you can't easily move it by hand.   Once you've slid the scope forward and back for proper eye relief with your length-of-pull (or if necessary moving the rings forward or back on the rail), then drop a plumb line and rotate the scope so that the vertical axis of the reticle matches the line.   Tighten down and torque accordingly.  Call it good. Walk away.

Plumb line.

Thanks guys and yes I meant centered rotationally. I got the Arisaka leveler and I'll cross reference with a plum bob.

That last bit is the fun part. It always takes me forever to get everything torqued down while keeping both levels, level.

I've yet to see a scope mount that doesn't want to twist the scope a little as torque is applied. I go slowly working all the screws maybe a quarter turn at a time once there is some pressure. If the scope moves, I back off and do it again. The Aero Precision mounts are a real treat.

Wheeler scope leveling kit. The nicer of the two they offer. One level for the barrel and one for the scope

Feeler gauge

feeler gauge and a wooden ruler. foolproof and flawless for ~$8

I am incredibly anal about my scopes being level. Therefore, levels alone don't work for me. Not precise enough, too much room for errror. I've found the best way is to level the rifle both horizontal and cant (left to right), and use feeler gauges between the base and bottom of the scope (flat section underneath elevation turret). If you can't find feeler gauges, sometimes a fresh deck of cards will work.

Double checking with a plumb bob is good too, but still tough to get perfectly centered.