OK, see is this helps:
what we'll do it set up a data validation list so you can choose your option for G2, then use a vlookup to return the corresponing % in G3 - no typing needed.
1) On your nitrogen sheet, go to insert | name | define.
2) Type Chemicals in the 'Names in workbook' box. Set the name to refer to
=OFFSET(NITROGEN!$B$1,0,0,COUNTA(NITROGEN!B:B)-1)
& press enter.
This will give us a dynamic named range that will grow as you add more entries to the NITROGEN sheet. The formula defines a range of cells that starts in B1, and goes down as many cells as there are items in column B:B. The -1 at the end is to compensate for the column heading. (It is imprtant not to have gaps within your data in col B).
3) On the IRRIGATION sheet, select G2 & go data | validation
4) set Allow to 'List". Set the cource to
=Chemicals
We now have a drop down list that (a) picks up values from your nitrogen sheet and (b) will grow etc as the list changes.
5) On the NITROGEN sheet, go to Insert | name | define again.
6) Set up another named range called LTable (for 'Lookup Table'). Set it to refer to:
=OFFSET(NITROGEN!$B$1,0,,COUNTA(NIRTOGEN!B:B)-1,2)
This will give us a dynamic named range that will (a) grow as the list changes, and (b) refer to the data in both cols b & c.
7) In cell G# on the IRRIGATION sheet, enter:
=vlookup(g2,NITROGEN!LTable,2,FALSE)
All going well, all this should end up returning the right %s for entries you pick from the list in G2 & should not require further maintenance as your data changes.
There are ways to further refine this. We could, for example, make the entries in the drop down in G2 depend on another cell - for example, we could set up a similar drop down for Manufacturer first...
Anyway, I hope I've understood what you wanted - see how you get on & post back.
Paddy