I'm curious Wolfy, why do you think yeast calculators like Mr. Malty are "virtually all guesswork with a large amount of error"
Do you have any kind of data to back up this claim?
What data would you like? That's actually a bit of the point that I am making - there is so little actual hard evidence and hard data that it's its impossible to either prove the calculators are correct, which ones are best or how far they are out they are. In addition each individual home-brewing yeast propagator will have different starting conditions, different growing conditions, and different results, but without counting yeast cells there is no way to know.
We are dealing with billions of live living organisms, they live, grow, reproduce and die according to how we treat them, because of this, I'd argue that it does not really matter if we pitch 450billion cells or 350bilion cells, or if we know if we have 450billion cells or 550billion cells - those are huge factors of error, but likely all produce equally good beer. So one's focus should not be on pedantic maths, but growing good healthy yeast, then as long as you are anywhere close to the volumes of starter, yeast cells and slurry volumes required, then that is all that is required.
As home brewers we do not have the facilities, equipment or ability to count yeast cells, what we are trying to do is make good beer, and what makes good beer does not necessarily relate directly to a specific number of yeast cells that are pitched, becasue other factors such as yeast health are equally important to the number of cells (reference for that is on the podcasts featuring Chris White).
Even then if you compare the yeast-calculations in Ray Daniels' book, to those in JP's book, to those in Jamil's book, you will find that their suggestions as to how much yeast should be used to pitch differs by factors of around 10x - and that is just working on how much yeast to use, not even how much you have!
I know of only one home brewer (and he's on the UK forums and not these ones) who actually counts yest cells, for the rest of us we are all dealing with 'educated guesswork'.
Unless you count yeast cells - like Jamil did to formulate his Mr. Malty calculator - there is no way to know what is actually happening, it is just 'guesswork', educated guesswork yes, but due to the many variables involved there is a large amount of error.
I trust that the Mr Malty calculator is a great basis for providing an estimate of how much yeast is theoretically needed - and I use the exact same calculations as he does on my own spreadsheet.
I also trust that the growth-calculations that the Mr Malty calculator uses are valid for the situation that JZ developed them from, for his yeast supply, for his wort, for his nutrients, for his stir plate - and that they give an excellent basis for all home brewers to work from. But that does not mean that by using them other home brewers like you or me will get the same number of yeast cells as the calculator says - nor does it matter what the exact number is as long as it makes good beer, which is why the calculator is a great basis but being pedantic about exact numbers is irrelevant with the level of error involved.
As examples of this level of error, JZ lives in the USA and most likely had his yeast supplied directly to him from the yeast-vendors and it would be very fresh. Having yeast shipped to AU from the USA and having it sit in shops or our home fridge has a large impact on the number of viable cells, so when JZ starts with xxx number of cells, here in AU as home brewers we are starting with an unknown number of lesser cells than him. JZ also (presumably) uses USA based wort products, but does he use extract, AG or what nutrients does he use, we don't know. The only way to reproduce his results is to follow the exact procedures he used, which we cannot do.
I could continue but I've written an essay already.
Yeast calculators are great, they allow us home brewers to have an estimate of how much yeast we need and an estimate of how much yeast we can grow.
However, without actually counting the yest cells, we do not know, or really care, or really need to know, how much yeast we have - and as long we have enough good healthy yeast to make good beer, it does not really matter what any calculations say!
If MrMalty says you need a 2.5L starter, I'd use a 2.5L starter, but that does not mean that you will have the same amount of yeast as JZ calculated, you might have more or you might have less, but if it is viable and healthy it will grow and reproduce and ferment your beer well, and when it does who cares exactly how many billion cells were pitched into the wort?