There is probably a good reason that the recipe calls for bittering units, instead of hops by the ounce at your 60 minute boil for challenger.
The challenger probably will be the predominate bittering hop for that particular recipe. Since each particular hop variety will have a range of alpha acids for any given years growing season and location of the farm, the alpha acid level will be different from one package to the next. I've noticed sometimes that I'll even get two different manufacturers hops of the same variety with different alpha acid levels during the same homebrew store shopping excursion.
The recipe designer probably felt that the amount of bittering in this particular recipe was critical to the final product and gave the challenger bittering hops as bittering units, to make sure that the bitterness was spot on.
The other hops may not be adding much bittering and are more for flavor or aroma. He may have felt that adding these by ounces isn't that big of a deal for this particular recipe.
I can tell you this much about my own recipe formulation. I usually pay more attention to bittering than flavor or aroma when it comes to hops in my recipes. My best beer recipes have always been the ones where I get the malt and bitterness well balanced for the particular style.
My late addition hops though, such as those that go in at 5 minutes are not the ones that I pay real close attention to. For example, if I need 1 ounce of fuggles at 5 minutes and only have .75 ounces in my inventory, I'm usually satisfied with the .75 ounces. The reason is that the alpha acids aren't really being utilized at only 5 minutes. At 5 minutes, it's the Beta units that are being utilized for that aroma/flavor.
Since the original recipe designer apparently felt that 20 bittering units of challenger was so important, I would measure those hops out accordingly to get the 20 bittering units. He probably felt that the challenger bitterness should be predominent in the bitterness characteristics of this particular beer.
Most recipes either go by ounces or bittering units. Most don't go by ounces and bittering units both, so I can see your reasons for having questions about the hopping schedule. Hopping based on bittering units is more accurate when it comes to final bitterness. Hopping by bittering units is not as accurate when it comes to flavor and aroma though.
Most of us don't pay as much attention to Beta as we do to Alpha. Maybe we should pay more attention to Beta with our late additions though. You've caused me to reconsider some of my recipes, where the flavor and aroma are important. I might start keeping track of my Beta more closely on those recipes, so that I can improve the flavor and aroma of the late hop additions.
This is my assumption, based on the information that you have given us. If you post the recipe here, we might be able to look at it more closely and see if this is the case.
