Focusing helicoid - how much beyond infinity?

[Hening Bettermann]

I have earlier reported on my problems focusing macro lenses at infinity due to a too steep pitch of the helicoid.
http://forum.luminous-landscape.com/index.php?topic=110587.msg930570#msg930570, reply #3.
Now I'm trying to specify measures for a custom-built helicoid. I go for a helicoid with a fixed camera mount (E-mount) and interchangeable short flange adapters for different SLR lenses.

These lens adapters should be optimised so that the whole system focuses at infinity - with some tolerance BEYOND.

Question: How large should this tolerance be? Will 0.5 mm be enough, or need it be 1.0? Not more probably? Or should it be spelled out as a percentage of the travel of the helicoid? And if so which one?

At this time, I have asked for offers from S.K. Grimes and http://www.adrianololli.com. If anybody knows of a good photo machinist in Europe, preferably Germany, please let me know. Griturn in the UK does no longer seem to offer custom building.

Thank you for your advice.

[DeanChriss]

I believe the tolerance depends on the materials used in the lens structure, specifically how much the length increases and decreases with temperature and the temperature range the lens must accommodate and still be able to focus at infinity. Because materials expand and contract a given amount for a unit of length, longer lenses need more "tolerance" than short ones.

[Hening Bettermann]

Thank you for your reply, Dean.