Ok, just cos i am bored at work i crunched a few numbers for you in beersmith. Its a bit of mucking about but since my records show that the bavarian lager kit works out at 21IBUs in 23L i can make a pretty good estimate. The dodgy part (IMO) is telling beersmith that you are only boiling a ceetain % of the extract and unless you weigh that out accurately, hop utilisation can vary quite markedly.
Anyway, i assumed you have 2 x 1.6kg tins. I then assumed you would use only 1/2 a tin in the 4L boil and add the rest at flameout. Note that i have used a evap rate of 15% since it son a stove top with an open pot.
20g of 12.6% AA Nelson Sauvin at 20 mins gives me 14 IBUs
20g of 12.6% AA Nelson Sauvin at 7 mins gives me 6 IBUs
So your total IBUs using this calc (and everyone who calcs this will no doubt get a variation but its a ballaprk figure) including the kit of bavarna lager = 41 IBUs.
Thats pretty high for something as light as a euro lager, its right up there with a czech pilsner.
Sure this is what you want?
If you took mfdes's advice and added 20g at 10, 5 and dry hopped you would shave 10 IBUs of this.
Its something that took my awhile to get my head around when brewing kits. You can easily over do the bitterness if you get a little generous with your hop additions.
Hope this helps
Cheers
DrSmurto
P.s. If you havent already bought the bavarian lager kit you could skip this, go 2 tins of unhopped extract and up your hops a touch and end up with 25-30 IBUs of real hop bitterness, not the isohop in the tin....... just a thought
EDIT - dry hopping adds 0 IBUs, only aroma. You could add another 20g dry hopping and gte the aroma you are after