What you're doing wrong is confusing causes and effects.

I don't know for sure why the new chain tended to autoshift, which could be related to worn sprockets, the shape of the chain's inner plates, FD adjustment, BB play, frame flex, or warped chainringds (for starters), but assuredly was not caused by the chain being too long. The length of the chain in the upper loop is fixed, and the tension is proportional to haw hard you pedal, having nothing to do with resting tension.

As for the "slack" when back pedaling. This again has nothing directly to do with chain length or tension. You get slack in the upper loop because of friction in the freehub or the chain running through the cassette and RD pulley system. The slack or sag happens because the crank is spooling the chain back faster than the rear drive will take it, and the chain is therefore transferring from the RD cage to the upper loop. The specific causes vary, and the source of the friction can be any or a combination.

So, stop fretting about the chain length. It was OK before, and as long as you measured big/big+1" minimum, it's OK now.

Deal with your backpedaling issue by observing and detecting the source of friction in those items I described earlier.