Hello. I've been trying to use awk to solve the following problem for some time with no luck. Was hoping someone here could give me some clues as to what I'm doing wrong.

I have a file that looks like this:

INPUT:

    >chr8:76290516-76290880
       578	T
       579	G
       580	A
    >chr14:22131464-22132025
       468	T
       469	G
       470	A
    >chr12:33695439-33695441
       468	T
       469	G
       470	A

Each record in the file has a header that starts with `>` I would like to print a new column which is essentially a line number, starting after the header which starts with  `>`, and to begin counting from the number following the `:` in the header. I would like each record (after the header `>`) in the file to be treated/counted independently.

I have tried to do this in steps, starting with adding the counts to the third column first, and then will attempt to add the header value to the counts following that. I have had no luck with getting the counts in the third column using this command below:

    awk '{FS = "/n"}{RS = ">"}{if(!/^>/){print $1, $2, NF, $3 }}' input.txt

DESIRED OUTPUT:

    >chr8:76290516-76290518
       578	T	76290516
       579	G	76290517
       580	A	76290518
    >chr14:22131464-22131466
       468	T	22131464
       469	G	22131465
       470	A	22131466
    >chr12:33695439-33695441
       321	T	33695439
       322	G	33695440
       333	A	33695441

Any pointers would be very much appreciated. Thank you so much!!

EDIT: I did not include my failed attempt to solve this problem, as @mensur pointed out to me, so I have edited to include. Thank you.
      awk -F '[:]'  '/^>/ {print;P=$2;X=0;next;} {print $0, (P+X);X++;}' input.txt