Dilution of wort and its effect on IBUs

Looking for a bit of guidance here. We are having an Augtoberfest party next month. I have 5 gallons each of a Marzen and Festbier but just found out the invite list is very large and I need to make about 10 more gallons of beer and was considering trying to do it on one brew day. I just don't want to make another lager (and not sure I have the time) so have decided to make a Kolsch instead. I use a RIMS system that utilizes a 10gal boil kettle and 10 gallon cooler for the MT.

Target:
OG 1.046
IBU 26

I plan to produce about 7.25 gallons of finished wort in my kettle. I usually have a longer settling period and then transfer via pump to my fermenter and leave about 1.25 gallon in kettle. I am planning to transfer about 3 gallons each to two different fermenters and then dilute with distilled to 5 gallons. I will use about 21lbs of grains (pretty much my max) and end at 1.072 and then after dilution end at 1.046 in each fermenter. My question pertains to IBUs and how to calculate the ending fermenter/finished beer IBU target. Does anyone have any suggestions here? I imagine if i target 26 IBUs in the kettle and then dilute the wort prior to fermentation I will lose some of the "hoppiness/bitterness" but how much? Any suggestions?

Your top off water will have 0 ibus. Resulting ibus after dilution would be initial ibus x (original volume / total volume)

In your example that would be 26 x (3/5) = 15.6

Cool. Thanks for that. Seems very intuitive, should have just thought about it for a minute I guess. My Kolsch recipe uses 2 hop additions. One at 60 and one at 15 minutes for my 6 gallon batch. I get about 21 IBUs from the first addition and then 5 from the second. That is getting roughly 80% of the IBUs from the first addition so I guess I'll just keep that the same. Seeing as I want 26 in the final beer, I will shoot for mine having 43 in the kettle, getting ~34 of those from the first addition and then the rest at 15 mins. Transfer 3 gallons to each and top off to 5 gallons.

I've never done this before. Do you need to boil the top-off water first? Or if I just buy 4 gallon jugs of distilled, can you take the cap off and just pour in? Worried about infection from that?

I'm confused on why you don't hop like you would if you were doing a full 10 gallon boil. Wouldn't that give you the right IBU after you dilute it with water?

I believe you could do that but there would need to be some consideration given to the hop utilization since boiling gravity would be different. I do think the amount of hops needed would be somewhat in the ballpark though.