Using a capo is like using a balancing scale.
Chord X – one that you don’t know or would rather “disguise” as some Open Chord Y, has a specific WEIGHT.
When you put a Capo on fret Z, you need to RE-BALANCE things by subtracting Chord X by the amount of frets Z to give you the new, much friendlier looking Chord Y.
Just start subtracting from Chord X until you get an easier one.
C#m – 1 = Cm. no good.
C#m – 2 = Bm. no good.
C#m – 3 = Bbm. still no good.
C#m – 4 = Am. Great! Slap the capo on the 4th fret, and BAM! You’re playing a C#m disguised as an Am.