I also think that boil off in terms of % doesn't work.
My batches are either 25, 40 or 50 litres (sorry I'm in the UK
) and the amount lost to boil off (in terms of volume) is the same in each case; regardless of batch size.
It took me a while to figure this out.....I usually make 40 litres and at first couldn't understand why my numbers didn't work out when making a smaller or larger batch.
The best thing to do IMO is measure the boil off per hour and then apply this figure to your recipes and adjust accordingly if you boil for longer than 60 mins.