Just for reference, Jasem's statement about this : github.com/indilib/indi/pull/859

So if I understand well there is a "sync" command in Indi to say the focuser is at a certain value.
I don't see a way to sync in OnStep firmware, is there one except the "zero"?

If I follow Blueshaw, an offset of max/2 would solve this issue.
Just for info, here my focuser set-up (than explains why I don't need the negative value or offset)
#define MinAxis4 0.0 // minimum allowed Axis4 position in millimeters, default = -25.0
#define MaxAxis4 38.0 // maximum allowed Axis4 position in millimeters, default = 25.0
and whith this set-up I have 38000 steps to travel these 38mm with 0 fully retracted. (48 step motor, and approx 20,8/1 gear)
When I start I set the focuser mechanically to zero (0), initialize the focuser to Zero and from then travel to approx half travel where my focus usually is and I'm done.

May be James has some ideas.