You want the diff to be completely or nearly completely full. Otherwise the gears will push it out of the say and it won't do much if anything.

Start with the empty case and large bevel gear installed and fill the bottom of the case with putty equal to the top of the bevel gear. Then insert the small gears pushing them in so that they displace as much putty as possible.Then full in all the spaces with more putty and add some oil to coat the surface of the putty.

There should at this point be enough putty so that excess will squeeze out a little when you install the other case half with bevel gear installed. I try to squeeze out as much as possible before separating the halves again. Now the bevel gear will likely be stuck in the putty (which is OK). The oil prevents the putty from sticking and you end up with a makeshift mold. Remove the excess putty (or add more) and joins the case halves again.

You should be able to close the case and you are set. If you need to remove more putty you can.

After using it once, you may need to add more putty due to voids that were not filled during the initial process.

My idea is to use weight as the measurement on a setup sheet. Or at least so that two diffs can be made equal. The putty sticks to itself so you can pretty much remove all of it from a diff and transfer it to another. I did this after messing up an outdrive.

Above is how I fill the AE diff which is very strong. The Spec-R internals are a little more fragile so more care might be required. You are better off filling these a little at a time rather than overfilling them.

I agree the TC6 is a great car. The Worlds will be a great race and we expect a good result as all of our top drivers will be racing.