Finding a specific cell contanining value greater than the desired value

I'm looking for a way to return a specific cell value from a range based on an initial desired value. It looks like this:

DesiredValue typed in cell: | 125|

Search Range:

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line1         RangeA|RangeB

line2         1     | 50

line3         51    | 100

line4         101   | 500

line5         501   | 1500

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I'm trying to get two formulas:

the first one returns the immediate bigger value in rangeA (500 in this case), the second one returns the immediate lower value (101 in this case).

Currently I'm using a very ugly if formula, for the greater value as an example:

if[DesiredValue]<=[b2], [B2], if [DesiredValue]<=[b3], [b3], if...

A modified version of this formula is used to get the lower number, but it is really not the way to do it since i'm gonna have to use it in a couple hundread ranges.

Thanks in advance!