The biggest "moving and semi-unpredictable part", by far, is the effects Khem entering the war (in army and income).
By contrast, right now, we have a reliable source of both increasing Treasury buffer and prestige buffer (and loaners who think our victory is inevitable). We should make use of that.
If we go into the ultimatum without enough Treasury buffer to support a war (and tank the effects of a partial blockade), it amounts to restricting our choices, and being forced to take that ultimatum.
(If that is your plan, then an unfavorable Khemetri trade deal should also be taken, to make their ultimatum less stringent. The more profits they make from trading with the People, the less they actually want war--and with luck it'll open up the Sacred Warding, allowing for further negotiations.)