I lock it in place then maneuver it into place. There is simply no way of knowing that it is in right at the time you do it. But, following a combination of the FAQ, and your own observations on how it came out, and the results of your inspection of the layout of the baffles in the tank you can get it to seat properly. To observe the tank baffles, use a flashlight, and keep it as far away from the opening into the tank as you can. Turn it on and off far, FAR away from the tank opening (this will keep any micro-sparks in the flashlight switch away from fumes).

An extra, unsolicited tip: When you are putting the big, plastic nut back on, to hold and seal the sender assembly in the tank, DON'T line up a piece of wood on one of the spines on the nut and whack it with a hammer. I think it mentions this in the FAQ, but don't do it. I nearly ruined a new sender unit this way. For months after that it tended to stick at the level it was at when I whacked the plastic nut. Now, finally, it moves freely again. But it really upset me for a while because it happened to a brand new, Bosch, sender unit. $$$$$
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1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny