I assume the 81 is 81M, not 81m.
you want to dilute it 81/2.5=32.4 times, which means one part acid, 31.4 parts water.
so what is a part: 675/32.4=20.83ml
so measure out 31.4*20.83333 parts water, or = 654.17 ml water, then slowly pour in 20.83ml of the conc acid,under a fume hood, stiring, and pouring slowly.
Put into a stock bottle, label it with the contents, storage requirements, and safety instructions, and chemical category (Corrosive).
Wash out your measuring cylinder, and put away your gloves,and safety glasses, stow all equipment and chemicals.