I have a recipe that is given only in percentages. What is a good reference for determining the total amount of grain for a specific style of beer? For example how much total grain for a porter or an IPA. I have searched so far with no luck in this area.
Having a recipe in percentages rather than amounts is actually a GOOD thing...but I'll explain why later.
You're unlikely to find any reference that will give you what you've asked for above - a guide to grain quantities by beer style. Sure, you might find something that will tell you what types of grains are used in particular beer styles, some general guidelines about their proportions and the range of OG and color for individual beer styles, but you won't find something that tells you exact amounts of each grain to use for a given beer style because the actual quantities of grains needed vary from brewer to brewer.
One of the major reasons that grain quantities vary between brewers is what is known as efficiency. That's a measure of the total amount of fermentable sugars actually extracted in the mash and sparge compared to the theoretical maximum amounts available in the grains. Some brewers get high efficiency numbers 85 - 90%. Some get low efficiency numbers - 60 -70%. Those that have lower efficiency will have to use larger quantities of grains that those with higher effciency.
That's where the advantage of the percentatges rather than actual quantities in a a recipe comes in. If you know your efficiency and you know the grain percentages, you can work out the exact amounts of each grain you'll need to hit your target OG. If you're only going by the weights provided ina recipe and your efficiency is different from the person that created the recipe, you're gonna get different OG numbers.
Until you know what efficiency your system has, you can generally assume that you'll get around 70%. The other alternative is to brew a batch using a recipe that sounds good to you and take note of your OG reading compared to the one published with the recipe. You can adjust grain quantites accordingly the next time you brew the recipe so that you get closer to the published OG....if that matters to you.
Alternatively, you can use a recipe that lists its assumed efficiency and compare it to your actual result. The difference between the publishe OG and your actual OG will help you determine your actual efficiency number.
Although you can work thos all out on paper and a calculator, brewing software makes it MUCH easier becasue you can easily see how minor changes in grain amounts effect the predicted OG...assuming that you know your efficiency.
