Question: Looking up VAT data from another table by code and returning the vat value to another field on a form

I have a table called VATCodes (first field has ‘Sage like’ VAT codes ie T1,T0 etc, the second field has corresponding VAT % rates ie 20.00%).

Form/Table 2 (test), references the VATCodes table, has a choice field on it (combo box style) and a vat value text box. What I would like to do is to populate the choice field (values) from the combo box then lookp this up to the vat table to return the appropriate VAT% Rate and enter this in the text field (or worst case formula field). Please note that I prefer to look up the VAT codes from the VATCodes table rather than ‘hard code’ the VAT codes into the choice field (if possible) to save duplication on both tabels. also i would prefr the vat vaule to be an edit box to write the valu einto the record at that time. so that if one went back and chnaged the vat table vat vaule to say 22.00% it ould copy that vaule over from that point in time leaving all previous records on 20.00%
hope that makes sense

Has anyone achieved this already and could you offer how you approached it please.