You'll have to be more specific about how you are producing your error and what the error message is. the ctl=null (if I remember correctly) is inside a loop so it's choking on attempting to clear the contents of a control that is likely locked, invisible or where the cursor is already in the control. You'll have to be very specific about how you are producing the error.
The ID field is in the table because I put a PK on every table I create, if you don't want it, don't append it when you append the other data.
As far as the rest of what you're asking, I don't understand.
If you're trying to find the X value for any pair of F/T values all you have to do is link the F/T values to your matrix table and add the X field to your query.
For instance if you had
Code:
tblTest
Test_ID Customer_ID PO_Number F_Value T_Value
1 1 10000 2012 3
You'd just link the F_Value and T_Value back to your matrix table to retrieve .0158