Hi Trev,
Sorry, the 25 should be 250 and it's yes, the first part e.g. 250ml/(1350ml+250ml) is all ml. Why are we adding 250ml on the bottom? That's your acid, the 1350ml is the water, giving the total volume of solution.
As your acid is already diluted to 81%, you multiply by 0.81 to get the final dilution. Hopefully I have remembered that right!