First of all, chain breakage on the road is relatively rare and unnecessary. Most chain breakage is the result of a chain not being closed properly in the first place, or the result of hard shifting under load which can push the plates outward beyond the end of the pin.

If you're riding a 8s or more system, you can't close the chain with your chain tool, so the best answer is to carry a connecter link. If you're chain breaks, use the tool to remove the rest of the outer link at the break leaving you two inner link ends which you join with the connector. The spliced chain is still the original length.

On the off chance you have to actually shorten the chain, the answer to your last question depends on how much downside room you left yourself originally. If you measured by the big/big +1" method, your chain started out at the minimum length, and you need to restore it to that length ASAP if not sooner.