Tracing out the boundary of the Tay catchment was easy. The watersheds are clear on the maps, everywhere except for an area west-north-west of Kirriemuir. It's the mystery of the Quharity Burn. This water flows out of the southern fringe of the Mounth, only to be lost in a bog and have its waters stolen and rustled into the Loch of Lintrathen (off the bottom left of the map below) and eventually the River Isla. However the burn and its name are resurrected a short distance further on, re-starting the journey from small beginnings towards the River South Esk.
Humans are probably the culprit here. The Inzion Burn, which steals the original Quharity Burn's waters and takes them south-west, is suspiciously straight. It looks like a channel deliberately cut to drain the bog. Newton Moss, at the head of the Inzion Burn, is still marshy according to the map.
So here's an example of where the watershed has been altered by human intervention. What implications does it have for my route? Should I follow the watershed as it is now, or as it should be?
The latter seems the more purist approach, but misses out a cracking hill: Cat Law (671m). It stands proud right at the edge of the Mounth and would make a great entry into the Highlands. Given its height, position and relative isolation I'll bet the views from the top are terrific. The purist approach would mean a less dramatic introduction to the Highlands, creeping in circuitously via the back door over the lower moorland whalebacks of Creigh Hill and Milldewan Hill.
Ultimately, pragmatism will probably win. If it's a fine day it'll be hard to pass up a visit to Cat Law. If the weather is rotten, the lower level route will probably seem more attractive.