Need clarification please...

I posted a thread on 3/11/2009 with a question about a recipe I am going to make that deals with late extract addition, hop utilization and Beersmith calculations ( https://www.homebrewtalk.com/f37/late-extract-addition-hop-utilization-108155/ ). BarleyWater listed a couple of thread links that I read on the subject and I did research this topic some before posting. So...my question, as simple as I can make it, is:

If I were to boil my hops in a low gravity wort, say 1.010 or so @ 3.0 gallons, and had increased hop utilization due to the late extract addition; when adding my top-off water (~2-2.5) am I redcing/diluting the IBU's?

I've thought of it as making a pot of tea, adding water to the normal strength tea would dilute all of the flavors, am I correct?

Any clarification on this subject would be enormously appreciated!

Of course adding water is going to reduce IBUs. I guess I don't understand what you are asking. If you have 50 IBUs in 3 gallons of wort and then add water to it you are going to dilute the IBUs.

BeerSmith is not calculating the IBUs in the partial boil. That figure it gives you is for the wort + top off water.

That is my question w/Beersmith, I entered my info into the program and it gives me the IBU's I want, however, I don't think it is calculating top-off water, because it would seem that with 1 oz of Fuggle hops (AAU 4.5%) for 60 minutes and .5 oz of EK Goldings (AAU 5%) for 15 minutes along witht he late extract addition would not be in the mid 30's for IBU's I did a little math from John Palmers How to Brew, and I got like only 20 IBU's from the 60 minute boil and another 6 or so from the 15 minnutes it is at the low end of IBU's for a Stout, I just dont think that these two hop additions will give me 36 IBU's.

I just played around with your numbers a little bit in Beersmith, and the one thing you aren't saying is exactly how much extract you are using early and late, but here's what I got:

2.5 lbs LME 60 min
1 oz 4.5%AA hops 60 min
4.5 lbs LME 15 min
1 oz 5%AA hops 15 min

26 IBUs

3 gal boil, 5 gal batch

Are you sure you have it set to a 5 gal batch? Which formula are you using (Rager, Tinseth, Garetz)?

Depending on what kind of a stout you are trying to make, that could be a perfectly acceptable amount of hops. In the end, make it and see if you like it, and adjust the recipe the next time you make it as you see fit.

yeah I have it set to 5 gallon batch size and have it as a 3 gallon boil with:
1 lb DME (not LME) 60 min
1 oz 4.5%AA 60min
.5 oz 5%AA 60 min
4.75 lbs DME (not LME) 15 min
.5 oz 5%AA 15 min

I just brewed this yesterday and made some adjustments, such as trying to get the best hop utilization I could w/3 gallons H2O, 1lb wheat DME and 1.5 oz hops in for 60 min, I also increased the Amber DME by .25 lb because I had a little extra that I didn't want to store. I ended up w/ 1.058 OG @ 75° ~ Actual OG 1.060. I was very impressed with White Labs Irish Ale yeast I made a 1000ml starter @ 7:00pm the night before and pitched it @ 2:00 pm yesterday, my airlock was bubbling away within 3 hours! Beersmith numbers just seem off to me when it comes to the IBU's. I'm not saying they are, it just seems as though once you add top-off water it does not calculate dilution of the IBU's. Again, I'm not saying it doesn't take that into account I just don't see how I can get IBU's in the mid 30's with a partial boil and only 1 oz of hops. Because of the roasted and malty flavors of a sweet stout I wanted to hit middle of the road (30 or so) with hops for a nice balance, I think I'm going to cold press some coffee and add it to the secondary for a little extra bump in the flavor. I'm probably wrong, but it seems odd for Beersmith to calculate such high IBU's with dilution. BTW Tinseth, just because it seems to be the standard and it is what Beersmith's default is.

I don't have any way to measure my actual IBUs vs what Beersmith predicts, but I think it is a good baseline to predict about how hoppy a beer will be.

The trick is that these are numbers based on a formula that is based on ideal conditions and observations. If you do exactly what Beersmith thinks you are doing, then you will probably get pretty close to expected IBUs, but realize that real world conditions (how old the hops are, how they were stored, how much DME REALLY went into the batch) will effect the outcome. And really, who can taste a 4 IBU difference?