A year ago(or more) when I first started Chair exercise I asked SP and was told - enter it under "low impact aerobics" and the time you spent on it!!
I now use a Heart ate monitor and find the calories are very similar to what SP estimated as "low impact aerobics.
However that applies to aerobic chair exercise - for
Strength chair exercise, or stretches, I enter it under strength and dont worry about weights and reps!! But if you enter the actual exercises you did (you can make a workout plan so you don't have to enter each one separately each timer) then you get more points in the SparkPoints section!!
Surprisingly Plates is under aerobic in the SP list (the Chair Core Strength DVD comes under Pilates)but I felt it was more Strength and stretches so I entered mine under strength training!!
I agree the fitness tracker needs to be made to cater for more than just "main stream" exercise.