Hi John
Yes you are correct I was thinking of individual spot age errors based on counting statistics. I have attached a spreadsheet which is a cutdown version of a speadsheet we used to use a long time ago. It had other worksheets in it and a macro to perform the iterative age calculation. I am not sure who created the original sheet, it was just handed down from person to person.
Captured in the cut down worksheet I have attached, is how the individual 1 sigma errors were calculated based on Pk/Bg counts, count times, contributions from U/Th isotopes etc. It doesn't contain any of the original macro for age calculation, but I assume you don't need that as you already have that in your software. All you need are Pk/Bg counts, count times, decay constants, and the "proportions" or contribution of Pb from the different isotopes to calculate the 1 sigma.
Unfortunately I am not an expert of chemical dating (or microprobe!) so I cannot say whether this is the correct way to calculate the 1 sigma errors for chemical probe dating...perhaps Mike or Julien could comment, but it appears logical to me. Anyway I understand its potentially a lot of work for little public demand so understand if you are reticent to put it in.
Cheers