Do you use a similar "low tech" method for hopping as well?
Believe it or not, I still work in Amateur Alpha Units (AAUs, a.k.a Amateur Acid Units) most of the time. AAU is the British acronym for Homebrew Bitterness Units (HBUs). I will calculate IBUs if someone wants the value, but IBUs are overkill in most recipes. There is one area in home brewing where going through the trouble of calculating IBUs is worth it for me; namely, cloning. However, I use AAUs within my home brewery for most of my recipes.
What is the goal of performing any calculation in a brewing? The answer to that is question is to give brewers a metric to use in brewing. IBUs are an analytical laboratory measurement of bitterness. All of the algorithms for calculating IBUs are approximations of what one would expect bitterness to be using that metric. AAUs/HBUs give us the same kind of metric just in a much easier to use calculation. Granted, there is more error, but it is a "good enough" calculation that home brewers used for decades.
AAUs can be tackled multiple ways when formulating a recipe, but the simplest way will be demonstrated. Let's use Anglo-American Bitter as an example. That beer was originally formulated using Cluster that had an alpha acid (AA) rating of 6.2% and Cascade that had an alpha rating of 5.6%. Subsequent brewings of the beer employed Cluster with a 7.3% AA rating. The cluster that I currently have on hand has an AA rating of 8.0% whereas the Cascade that I have on hand has an AA rating of 7.6%. There is no way that I can do a one-for-one substitution with these hops and not produce a beer with a different character. I can either sit down and calculate the IBUs added by each addition and then determine the quantity needed to hit those numbers using the new hops, or a I can use AAUs, which I will refer to as a quick-and-dirty method.
AAUs = amount * aa_rating
AAUs
60 = 1 x 7.3 = 7.3 AAUs @ 60
AAUs
10 = 0.5 x 5.6 = 2.8 AAUs @ 10
new_amount = old_AAUs / new_aa_rating
new_amount
60 = 7.3 / 8.0 = 0.91 ounces or 26 grams
new_amount
10 = 2.8 / 7.6 = 0.37 ounces or 10.5 grams
If one runs the original AA ratings an quantities and the new AA ratings and quantities through an IBU calculator, one will discover that the values are pretty darn close. Herein, lies the power of AAUs. There is no need to power up a computer. AAU calculations can be performed using pencil and paper.
Remember, any bitterness equation is not going to tell a brewer exactly how bitter a beer is going to taste. There are lot of variables in that equation, not the least of which is the yeast culture employed in fermentation. The only way to know if a beer has the correct bitterness level is to brew it a couple of times while tweaking the additions, and then re-brew the experiment that produced the correct level of bitterness to ensure that it was not a fluke. From that point forward, AAUs will provide a brewer with a fast way to adjust hop additions based on the AA ratings for any given hop.
Using this method and the PPG-based method I outlined earlier, a brewer can scale any given recipe to his/her brewery very quickly. To scale hops up or down in batch size, one just multiplies the AAU values by the target batch size divided by the original batch size. While hop utilization improves with volume, this scaling method will hold for most homebreweries.
scaled_AAUs
time-t = recipe_AAUs
time-t * target_volume / recipe_volume