Measuring filter offset - How do you do it?

[Corpze]

Hi!

I am wondering how you are measuring the offset between your filter to get good and reliable readings.

I have on two different occasions measured each filter five times, and noted the "final focus position" in the autofocus routine in sequence, and then compared them with my L which is the reference filter.

Even if I take five readings of each filter, and average them out - i still don't get perfect focus compared to L.
This routine takes about 30-45 minutes - can it be the temperature which changes the focus point during this time?
To my knowledge - there isn't any other way to do this right? I know that N.I.N.A has a built in "filter offset" measuring routine.
There would be one way to work around this "problem" - new focus for each filter change... but the only setting in Sequence I have come across is "Focus every XX minutes"

How are you measuring your offset?  

/Daniel