t(20th letter? Better use a=1,b=2: t=20, +13=33, 33-26=7→g; n=14,+13=27-26=1→a; z=26,+13=39-26=13→m; y=25,+13=38-26=12→l; l=12,+13=25→y → "gamly")
j=10+13=23→w; w=23+13=36-26=10→j; l=12+13=25→y; a=1+13=14→n; q=17+13=30-26=4→d; y=25+13=38-26=12→l → "wj yndl" tnzyl-syt-jwlaqy
Atbash result: gmabo hbg qdozjb → not readable. ROT13: a↔n, b↔o, etc. t(20th letter