Where the PTI should be reduced:
Get rid of that big nasty horrible hole in Central Asia! The area was both well known and well explored from both East and West. The only place the PTI should remain there are in the inaccessable mountains of the Hindu Kush, Himalaya, and Tien Shan ranges. These should have a few passes through them, however, to simulate the Khyber Pass, the Ferghana - Kashgar route of the Silk Road, and so on.
It's fine if most of northern Siberia remains PTI. Even if it was explored, it really wasn't of much use for anyone. The "Siberian Corridor", which will hopefully no longer be a corridor but merely mark the southern edge of the PTI, needs to have its western end moved much further north, however, as the Khanate of Sibir was due east of Moscow and Perm, not down around the southern edge of the Ural Mountains. All of European Russia should be uncovered, as well.
Most of the North American PTI should disappear as well, as the Plains region was fully explored by 1820 (and long before). Parts of the Roickies should remain inaccessable, with a few passes here and there - near the headwaters of the Columbia in British Columbia/Alberta and the Missouri in Montana/Idaho for example where explorers had breached the mountain range. Mackenzie had explored much of northwestern Canada near the end of this time period, although I don't know whether this makes such remote areas worthy of emerging from the PTI.
The South American PTI is pretty reasonable, although a little more interior exploration in southern Brazil might make sense. The rest represents impenetrable mountains and jungles anyway, and weren't explored until well outside the EU2 time period (and some areas remain unexplored to this day). The same goes for the Australian PTI; maybe add the rest of the seacoast provinces, but the Outback wasn't explored until well after 1820.
The African PTI will need some adjustment. I assume that the Nile valley down to the Sudd swamps will emerge from the PTI (even the ancients had knowledge of the Nile as far as the Sudd) so that Nubia and Ethiopia will be able to take their correct possitions. The Zambezi valley up to Zimbabwe should be added as well, as should the uper Niger river valley in Mali. The main connection between Mali and the rest of the world at this time was through the Sahara, not through the inhospitable terrain to the south, so the PTI should have at least one corridor (Mali - Morocco, historically the most important route) through the Sahara, if not two. A PTI corridor to the coast of Senegal and/or Nigeria and/or Ghana wouldn't be all wrong either, but the Sahara connection was the most important in the EU2 time period. Beyond this, the rest of Africa should remain PTI, as any outside explorers almost immediately dropped dead from one or another tropical disease as soon as they set out.