The 'connectionLength' link anchor function places the anchor of the link at a point a specified length along reference link (counting from reference link's source). It accepts one argument, which can be passed within the linkAnchor.args property:

length	number	The length at which to place the target anchor. Default is `20`, meaning a point 20 pixels from the source of the reference link.

Example:

link.source(link2, {
    linkAnchor: {
        name: 'connectionLength',
        args: {
            ratio: 50
        }
    }
});