BGP Load balancing
Various methods can be used to achieve load balancing using BGP, including attribute manipulation, multipath load sharing, and additional paths. However, it is not possible to achieve a specific percentage split using these features. It is a manual process, and by knowing the prefixes that are out there, you can choose which prefixes to route in which direction.
However, you can use Cisco Performance Routing (PfR) to achieve a more specific routing distribution, based on dynamic values, and particular performance metrics.
Links
https://networklessons.com/bgp/bgp-attributes-and-path-selection
https://networklessons.com/bgp/bgp-multipath-load-sharing-ibgp-and-ebgp
https://networklessons.com/bgp/bgp-additional-paths
https://networklessons.com/cisco/ccie-routing-switching-written/cisco-performance-routing-pfr