The general geography of Eregion is well established, but less certain is the location of its capital of Ost-in-Edhil. We do know that there was a main road that ran from Khazad-dûm along the Sirannon, and indeed that this was the 'only road of old to Moria from the west' (The Fellowship of the Ring II 4, A Journey in the Dark). This road would, by implication, lead to Ost-in-Edhil, probably placing the city somewhere on the Sirannon, or possibly further west, somewhere on the banks of Glanduin.
It should be emphasised that we do not know the course of the high road for more than a few miles from the West-gate of Moria, and for all we know it may have suddenly changed direction and led off to some entirely different location. However, given the convenience of founding their city on a river, and the general direction of the road as far as we know it, it does seem most plausible that the Elves of Eregion built Ost-in-Edhil somewhere close to the point marked on the map.