Hola ovny,

I like the idea about matching the trajectory between my different cartridges rifles. So, in my opinion, a good partner for a 8x68 would be a .270 Win. Even more than the 7x64 because this was designed with and for relatively heavy for caliber bullets. And the .270 Win is a universal round to be found everywhere. The .270 fills the gap under the 8x68 somewhat better than a 6 mm. Just my point of view.
By the way, where do you live in Spain? Now I have a son in Madrid, working as an economist in the Central Bank, and a daughter in Barcelona, also an economist.

Best regards