When I manually perform the data reduction I generally simply follow the formula from Longerich et al (1996). This formula is also employed by the common data reduction software (e.g. Iolite, Glitter, etc.).

If you look at page 902 of the paper you can find the formulas for the data reduction. 

Best

Article Laser Ablation Inductively Coupled Plasma Mass Spectrometric...

Here is a recent paper that outlines a standard workflow and recommended treatment of LA-ICP-MS data.

All the best

You can also use data reduction program for time-resolved U-Pb analyses called UranOS. This program is very useful and makes the data reduction including (i) proper visualization of the spread of raw data, (ii) optimizing the selection of time slices (iii) filtering the outliers, (iv) correcting the mass fractionation and (v) the drift through measurement session. 

You can download it http://www.sediment.uni-goettingen.de/staff/dunkl/software/uranos.html 

Best wishes

I would suggest you to use PepiAGE. If you find this software interesting, kindly ask me for a detailed manual to use it.