Work it up and give the ruler +1 prestige for each duchy in his realm too (or more, if you wish). This would make a "royal" position actually more prestigious, and would stimulate the ruler to create several duchies. Also otherwise, if all regions of a realm are in one duchy, that would be one prestigious duke (which may be, somehow, an exploit... although I don't know how yet. Breaking records maybe?)
If it's +1 prestige per region for the duke, make it the same for the ruler. Since the ruler rules all the regions, he/she should naturally have higher prestige than the dukes.
