One thing to remember here, is that many people have centered these crosshairs over Mount Diablo, when by November 9th 1969 the Zodiac Killer had never referenced Mount Diablo in any of his communications. We would have to wait until June 26th 1970 for the Zodiac Killer to send us the Phillips 66 Road Map with his crosshairs centered over Mount Diablo. These crosshairs had the numbers 3, 6 and 9 around its face, also suggestive of a clock face. So, although the Bus Bomb Letter crosshairs came with no indication of where they (or a clock face) should be placed, it is apparent in hindsight they were to be placed over Mount Diablo. There is also no indication in the Bus Bomb Letter that we should try in any way to rotate the five X's around to suit our needs. Below, I have placed a clock face over Mount Diablo to locate the positions 6, 8, 9, 10 and 11, marked out by the Zodiac Killer
I don't know whether these examples highlighted were the intention behind the five X's on the circumference, but it can be strongly argued, based on the Zodiac Killer's introduction in the Bus Bomb Letter, that he was threatening more attacks without announcing the murders he was to accomplish. The statement next to the crosshairs of "PS. Be sure to print the part I marked out on page 3 or I shall do my thing", suggests that these five X's would be the result of non-compliance, rather than the attacks he had already carried out. The Zodiac is split into 12 signs just like a clock face - beginning at zero degrees longitude - which is maybe why the Zodiac Killer attributed zero to the north face of his crosshairs on the June 26th 1970 Phillips Map. Had the Zodiac Killer originally intended to commit 12 attacks when he began his murderous campaign of terror, thereby completing the circle he had created for himself? Something to which he would later realize was simply unachievable.