Wow...I logged onto this site to post this exact same problem. Almost identical.
2010 IS250 F Sport
60 month service / approx 70K
Average fuel went from about 11 to 12 L/per100Km. Same conditions
I had fuel sensor recall done a few months back
One main difference is that I went to a Lexus "specialist" not a Lexus Dealer (half the cost).
I use shell 95ron. I used to use 98, but found no difference in economy and it's cheaper.
In terms of measuring the fuel, i use the trip computer as a guide (which is not accurate), but I measure it off an app where I log the km's, litres in and price. I always fill up to full, so i know my numbers are accurate.
I am only on my 2nd tank of fuel since service, so it could be too early to tell (ie maybe they put an additive in which gave me less economy on the first tank). I have only just started on my 2nd tank but the consumption at this early stage looks higher than normal