Human mystic base wit 20
+4 wit from tattoo
+2 wit from tallum robe set
-> 26 wit, which gives 34% bonus to base res
So 20 (base res) * 1.34 = 26.8
Cap of max +20 does not apply here, since it is only 6.8 bonus you get.
So you have a 26.8% res. Don't know if it is rounded up or down or used exactly as is to calculate the number of exp you get back, but anyway it's "about a 27% res" you have :)