Add Formulas to a UDF

Hi everyone I have created a couple UDF for minStock, maxStock, CriticalBusinessVol... So we need the User defined Fields, to work with each other. For example: (AvgsalePerMonth x 3)+ CriticalBusinessVol = StockMin) So we basically need flexible fields who always when a related field changes, (Outgoing or incoming Products) adjust itself with a formula... Does anyone have experience in that? Appreciate, Florian