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.