I combined the Tinseth equation with empirical hop utilization ratios from the following link from Palmer to develop my own simplified equation for IBUs, shown at bottom of this post, including estimation of IBUs from so-called hop stand or whirlpool additions as a post-boil high temperature steep. This is only an estimate, good within about 10 IBUs (kind of like Tinseth). An estimation of IBUs is better than nothing IMO.
Palmer:
http://howtobrew.com/book/section-1/hops/hop-bittering-calculations
Looking at Palmer's hop utilization ratios for a normal gravity beer of say 1.060, increasing boil time from 60 minutes to 90 minutes improves utilization from 21.1% to 22.6%. Wow, what a huge gain.
So here's my swaggy formula which you should find will get you really close vs. Tinseth, or as measured in a lab, within like I say about 10 IBUs, i.e., "close enough for most intents and purposes" (another one of my mottos). If you have multiple hop additions, you have to calculate them all separately then add it all together to get the total obviously.
IBU = oz * AA% * [sqrt(5*Boiltime)/V + sqrt(2*HStime)/V]
where:
Boiltime and HStime (Hop Stand a.k.a. Whirlpool time) need to be entered in minutes of course,
V is volume in gallons,
At high gravity (e.g., >1.080), change the sqrt 5 to a 4 instead, and sqrt 2 to 1.5,
At low gravity (e.g., 1.035-1.040), change the 5 to a 6, and the 2 to 2.5.
Maybe this will help somebody, and/or reduce the mystique with how IBUs are calculated. A little more messy basis stuff from yours truly is linked here, for anyone brave enough to attempt to understand it:
https://live.staticflickr.com/7891/45991029004_df99d89bc1_o.png