attempting to dynamically define table with index

....i'm almost there...think i'm missing something simple here but been at this for awhile now and would like some help.

Basically, I would like to replace the 4:8 in this statement with a dynamic range....users will be adding rows above and growing the curent table and I want the calculations on row 17 (attached document) to adjust for a moving/growing table.


=INDEX(4:8,MATCH(B17,INDEX(4:8,,12),1),10)+(B17-INDEX(4:8,MATCH(B17,INDEX(4:8,,12),1),12))*G11

I've tried the following:
INDEX((MATCH(">>begin<<",B:B,0)+2) : (MATCH(">>end<<",B:B,0)-1),MATCH(B17,INDEX(4:8,,12),1),10)+(B17-INDEX(4:8,MATCH(B17,INDEX(4:8,,12),1),12))*G11

Basically i've sanwiched the table between two markers (>>begin<< and >>end<<) and i'm trying to use match to find the markers to dynamically define the tables rows...

any help out there?