Automatic discounts - manual modifier best price

Hello

can someone explain me on how to setup the following,

if an automatic discount applies eg of 20%  , because customer always gets a discount

but then the customer setup up a ask for promotion of 30 %

-> only one discount may apply, ie best price, so meaning the ask for promotion

this hower then indicates we have to setup hte ask for promotion as being an automatic modifier

and how would taht work if we would setup a manual modifier for this?

so a manual modifier of 30% , ? how can that work in combination with  an automatic discount?

thanks,

els