Vlookup from the bottom problem
 

I have a spreadsheet that looks at frequency response of a filter and am
looking for certain frequency points within. For ease in explaining: The
frequency response of this filter is 851-869MHz. In column A I have the
frequency, in column B I have the rf level. Manually what I first need to do
is find what row 851 MHz is on, then I need to perform calulations on the the
adjacent power level. So if the level at 851 is +23dB i need to add -3 and
-65 to get 22dB and -40dB respectively. Lastly, I need to find where those
points are and get the adjacent frequency. I also need to perform the same
functions at 869 MHz. Needless to say, I can get all of this to work using
vlookups except when looking for the 869 info. Vlookup seems to look in
random places. I have used the -1 tag within vlookup to get the function to
start from the bottom, but no lock. Any other ideas I might be able to try?
I can provide the spreadsheet I'm using for testing if needed, as it may be a
little difficult to understand with such a brief summary of my problem.

Thanks to all!