problem in table control

hi experts,

i have a issue regarding table control.

i have created a table control which have fields ,po quantity and quantity sent to vendor, when user enters some quantity in sent to vendor field , bal quant is calculated n displayed , but when after entering the sent quant user press enter or save the proogram triggers from line 1 and agin the bal quant is calculated this resets my earlier calculated bal qaunt, but i do not want this to happen , i want that i my prooram bal qaunt should be calculated once only , how to do thsi can anyone help

thanks in advance

regards sandeep