formula to decrease a margin, in connection with increasing basic value

Thank you all!

In the meantime I googled out the vlookup function, but I still couldn't understand how to use it to suit my needs. Your replies made everything clear - mostly the file from Cheeky Charlie, but I want to thank you all guys for your replies.

In case anyone has this kind of problem in future, hereby I present the solution that works for me (see: attachment).