Single Row issue - VLOOKUP referring back to row n -- App creating a new row by default

Hi All,
I am working on my first AppSheet project. I have some coding experience from the games industry a few years ago (mainly python and javascript), but this is my first experiment in AppSheet.

I am trying to convert a Shrimp Yield calculator written by one of our geneticists into a simple 3 page app.
Page 1 - enter the parameters that give you the last harvests results
Page 2 - summarise the performance of that harvest
Page 3 - give options on what if… you change the % survival or % growth rate or both together

I have converted the supplied excel file to google sheets and streamlined a lot of what he had done, but he is using VLOOKUPS which refer back to the first row. When I create the app, by default it creates a new row for new entries as you would expect. But the lookup still refers back to the original row.

I have had a good look though the help, but I think I am turning over the wrong rocks.

I can post the google sheet if that helps

Any suggestions would be greatly appreciated.

Thanks in advance,

Jim