Making up caustic soda (NaOH) solutions can be quite dangerous, especially in large quantities. The dissolving generates heat and there is also a danger of splashing it on you while stirring, etc. I hope you know what you're doing, If you don't, ask before you do it.
Once you get the hang of it, about the simplest and most accurate calculation method is to use a chart like this.
http://www.handymath.com/cgi-bin/naohtble3.cgi?submit=Entry
On the chart, find the specific gravity of 8% NaOH at some temperature. I usually use 20C because it's close to room temp. On the chart, at 20C, 8% NaOH has a S.G. of 1.0869 g/ml. Therefore, 1 liter (1000 ml) of this would weigh 1000 x 1.0869 = 1086.9 g.
I assume you want 8%, by weight (w/w). Therefore, to make up 1 liter of 8%, you need 1086.9 x .08 = 86.95 g of NaOH.
To convert 80 gal to liters, multiply by 3.785. This gives 302.8 liters. Therefore, you need 302.8 x 86.95 = 26,328 g of NaOH. Convert this to pounds by dividing by 454. Therefore, you need 58 pounds of NaOH to make up 80 gallons of an 8% NaOH solution or, 0.725 pounds per gallon.
When making this up, first dissolve the caustic in about 70 gallons of water. After it is dissolved and cooled down, bring the volume up to 80 gal with water. Don't start with 80 gal of water, or you will find that the final percentage is less than 8%.
