"A" aggregation comes from B and C such that A=B+C and there is a need to predict these variables as a percentage of A. This is done to check the increase in A forecast is due to increase in B or increase in C. B and C are modeled as a time series.

Questions:

1. Can the same window size (100 as determined in the previous step) be used to predict B and C as a percentage of A using ARIMA?

2. Since B and C are expressed as a percentage of A, is it ok to predict B and then calculate C as (100-B) as time series B and C will be mirrored, meaning the same ARIMA parameters will hold good?

Any thoughts and recommendations?