“The country” is going to vary a lot depending on which territory you end up controlling, so I can’t give a standard answer. How much Theurgy you’ll be tempted to use per acre depends on the quality of the land and the total population (on and off-farm) you’re trying to keep alive, and all of that is going to vary tremendously based on your domain.
So let’s talk about the whole continent.
A maximally efficient new blood tax system would on my current assumptions be able to extract blood from roughly 60m people per year Hegemony-wide (out of the 110m population over 14. If that seems low, recall that historically it takes generations and lots of habituation/institution-building to be able to annually tax more than half of your population, and a fair few countries even today don’t have meaningful systems in place to get close to that proportion).
Coincidentally (really – it follows from unrelated assumptions in another bit of the spreadsheet), 60m is also roughly the number of people you’d need donating in order to generate extra harvests sufficient to feed the total current population of the continent.
However, that wouldn’t leave enough blood for the other vital aspects of the current Theurgic agriculture system – rain/irrigation, post-harvest drying for long-distance transport, barge-driving – which at current levels would require finding another 75m people to tax. (Or 662,000 per year to execute, which as discussed above is an order of magnitude beyond what you could expect any “justice system” worthy of the name to yield.) Moving all that grain up into Karagon costs a lot of blood.
So if you somehow end up trying to run the whole continent-wide empire on a blood tax basis, it would still come with massive dislocation of starving refugees, as people were forced to move from food-deficit areas (like most of Whendery and Karagon) to naturally well-watered surplus areas like most of Shayard. There would be a lot of people willing to give their loyalty to a rival leader who promised less starvation and dislocation through a continued system of Harrowing. All you need is a sacrifice of 1.3m people per year to fuel the continent’s whole agriculture system…
If you’re not trying to rule the whole continent, then odds are good you’ll be basing yourself in a food surplus area like Shayard or Aveche. That should leave you able to feed your demesne on a blood tax and (depending on the scope of your realm) you’ll also be able to use blood for other purposes–like fending off the child-sacrificing or refugee-sacrificing blood mage neighbors who are coming to steal your grain.
Each game should give opportunities to develop and define the new doctrine, yes. Maybe especially Metropolis and Omphalos, as you’ll be having formative engagement with the institutional priesthood during those games.
Not telling. 
If you don’t count the Eclect cult, the cult of your personality can properly begin this game, among the followers awed at your return from Xaos. I’ve been promising that for a long time.
“I’ve got the Epistle of Alexios right here and it’s totally a part of the Shayardene Codex…”