I'm sure there probably is, but I just stuck the numbers in a calculator.
Come to think of it, the PPG is probably listed on a website somewhere where you might buy it online, or you could use ProMash or something like that to find the PPG. Once you know the PPG, how much your adding, and how many gallons - the math is pretty simple. Rice Syrup Solids have a PPG of 40.
8oz = 1/2 pound
1/2 of 40 PPG = 20 Points of gravity from 8 oz
20 points of Gravity spread over 5 gallons means that your 8 oz of rice boosts the gravity of your wort by .004
.004 is ever so slightly more than the ABV boost, your ABV boost is actually somewhat less than .004 ( 0.4% ) but not so much that it matters or makes any difference - for all practical purposes the last 2 numbers of your OG measurement will be the ABV of your beer when its done.. eg your OG is 1.050, you can expect approximately 5% ABV beer
thats not exactly 100% accurate because actual ABV will be slightly less than that number depending on how big a beer you make in the first place, what kind of yeast you use, stuff like that - but its a pretty good back of the envelope type estimate, and if you're talking about an amount as small as 8oz its going to be so close that you could consider it "exact" to any degree of precision you'd be capable of measuring
Obviously if you made a really big batch of beer, much bigger than 5 gallons or were talking about a lot more fermentable material than 8oz of 40 PPG you'd notice the difference, but with only 8oz and 5 gallons - .4% is it - only have 1 significant digit of precision to work with