Backfocus on WO Z61+flat61A

[tan_21]

Hi,

I have a WO Z61 with a Flat 61A adjustable Flattener. How do I calculate the correct backfocus? My sequences are:

1) Z61 + 61A (locked at zero) > M42/M48(16.5mm) > M54/M48(2mm) >ZWO  EFW2" (20mm) > 11mm camera ring > 6.5 mm Image Sensor ZWO ASI1600MM Pro = 56 mm. 

2) Z61 + 61A (locked at 11.7) > M42/M48(16.5mm) > M54/M48(2mm) > ZWO EFW2" (20mm) > 11mm camera ring > 6.5 mm Image Sensor ZWO ASI1600MM Pro = 67.7 mm. 

Which one is correct? or neither of them are correct. 

Thanks in advance!

[PEMS]

I think for the Flat61A the first sequence is correct. Even that reads as 1mm too much and seems nothing you can do to reduce it.

Will say that WO seemed to change the numbers at some stage and I am presuming the Flat61A is sort of the original where the distances were from the Flat61A at 0mm. Then you added and set it up for 55mm from that fully would in position.