IBUs are not as difficult to estimate as you might think. All I've had to do really is to memorize three magic factors: 3.6, 1.6, and 0.6. They all end in ".6" so that keeps it easy for me. Here's my method, I like to call this "the Taylor formula", which is based on a practical approximation that will get you super close to Tinseth's fancy equations. This calculation is designed for pellet hops used in 5 gallons final boil volume, 60-minute boil, @ approximately 1.060 OG, though adjustments can be made for other things explained later. And away we go...
3.6 * oz * AA% = IBUs from bittering hops added @ about 60 minutes left in the boil
1.6 * oz * AA% = IBUs from flavor additions @ 10-15 minutes
0.6 * oz * AA% = IBUs from aroma @ 5 minutes
Add all these together, and then add another +1.5 to the final total to get the final grand total IBUs.
For higher gravity worts (e.g., >1.075), the primary factors are reduced somewhat to about 3.0, 1.3, and 0.6 (stays same). For low gravity worts (e.g., <1.045), the factors are increased to around 4.0, 1.9, and 0.6 (this third factor always stays same). So yeah, you need to use a little swag and finesse. But it ain’t hard. (That’s what she said.)
For different batch volumes (V) other than 5 gallons, you need to multiply the result by 5/V. If you use whole hop cones, then multiply your final result by 0.9 (a.k.a., 90%). No big whoop.