Except that's not the whole answer, because on top of the basic charge for simply doing the post transaction, there is an additional charge on a per byte basis which increases the overall cost
It is close. My estimates were for a 'moderate' size post or comment of 1-2 KB. For larger or smaller the numbers will vary. The per byte charge is pretty low and dropping, so the effect of size is becoming smaller.
And just as importantly, we don't know what it will be tomorrow because one of the major witness values is not only what the transaction cost is but what the per byte cost is
These aren't currently adjustable by witnesses. The rules are fixed (absent a software upgrade), although the prices are determined by market/usage conditions so in that sense you can't know in advance, only estimate. C'est la vie.
the 1/10 cost declaration from the witnesses is very likely to slide up over the next month or two
The pool formulas have a half life of a week, and they were pre-filled to 90%. As such it won't take that long. There is still some adjustment going on as things balance out but it isn't enormous. When the 1/10 as applied there was an immediate dramatic effect (15 SP accounts were limited to 1-2 comments previously). There will be further adjustment to equilibrium but only around the edges. For now it looks like the most significant change will be a smaller cost per-byte, reducing the importance of post or comment size.
I suspect that you probably can't get along with less than 20 to 25 SP
It clearly depends on what you are trying to do. The same numbers won't work for everyone. (Also some differences for different apps.)