alright, i see what you're doing here. and after re-researching for confirmation of my initial thoughts i think i can straighten some things out.
you're looking for 1.5% so you can match the concentration found in excel and dose accordingly.
from here...
http://www.seachem.com/Products/product_pages/FlourishExcel.html
almost word for word the directions you posted, wes.
the only lab test results i've found to date state that's what excel uses percentage wise, so it makes sense.
however, the calculations i still think are definitely off. since the aqueous solution you're referring to is a 50% concentration you'll need 3% of the total volume to make a 1.5% solution. so the final equation should = 3/100 or a 97 to 3 ratio.
beings that means we'd need 3oz to every 97 we'd divide by 3 to get 1 to every 32.33333.... a one ounce solution should make 33 and one third ounces at 1.5% given it's a 50% solution.
to check the math... 33 and 1/3 times 3 = 100 (% and/or oz)
that's multiplied by three our one ounce of solution would be multiplied also to make three. that's out of 100 so can also be referred to as % and/or oz. considering your bottle is a 50% solution you can divide that by 2 or multiply by .5. that makes your 3oz/3% into 1.5% and indicates how much gluteraldehyde is in the total solution.
you can use the oz measure in ml/cl also. it's based on 100% so it's transferable.
either way, a gallon is 128oz (at least here in the u.s.). if you were to do 97oz of water and 3 oz of glute you'd need to remove 31oz before adding the glute. that's 3 cups, 7oz.
this also means as i stated earlier in this post you can go by calculations meant for excel for other things as well. such as for killing algae/disinfection/etc..
feel free to check and double check my math.
hope this helps.