Chris pointed out to me that I mis-read your question.....
I will continue the way I started
we have 5n-2 gives the nth term
253 = 5n-2
255 = 5n
yields n = 51st term for 253 20 terms before this would be 31st term
5(31)-2 = 153 = 20 terms before 253
I think Chris answer may be the 19th before 253
if you use his equation the tem that is ONE before would be 253 - 5(1-1) = 253 which would be incorrect.