Best way to fill transmission with lube?

I have always done this the hard way, as described in the workshop manual. It is difficult to gain access to the filler plug below the differential, and having the correct tube to create a right angle turn, etc., etc.

In addition, I have read several threads suggesting that one can fill the gear box from the transfer case, recognizing that there is communication from the transfer case to the gear box.

Is there concern about adding gear lube into the transfer case, and allowing the gear box to seek its own level as fluid finds its way around the main shaft into the transmission? And rather than measuring the correct amount, what about simply removing the filler plug under the differential, and adding fluid to the transfer case until gear lube starts to flow out of the filler plug (and hence achieved the correct level)? Will this lead to too much lube in the transfer case?

Thanks.

Jim S.