Leave your batch size for the malts at 5 gallons so that it can calculate the gravity and color properly.
For the hops, you are probably ok to put 5 or 6 for the volume. From what I have seen in other brewing software, hops utilization doesn't get affected by gravity until you get up around 1.06.
If you are boiling 6 down to 5, just put in 5.5 as the average gallons.
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