Hello Mohamed,
It seems there is several limitations for the maximum possible backlash value. First, the driver software restricts the number of steps you can set to 512. Second, the focuser firmware itself restricts the value to 255.
As a workaround, you have two possibilities:

-As in the case of the Celestron 9.25 the CFZ=220µm, you could reduce the micro-stepping to 1/8 or 1/4 instead of 1/32 and thus reduce the backlash value to set in INDI tabs by a factor 4 or 8.

- Otherwise, you could try to modify the lines 918 and 224 of the firmware 304 as following:
backlash_count = myfocuser.backlashsteps_in -> backlash_count = (myfocuser.backlashsteps_in * 10)
backlash_count = myfocuser.backlashsteps_out -> backlash_count = (myfocuser.backlashsteps_out * 10)
Doing that, the value set in INDI is multiplied by 10.

Clear skies,

Pierre-Yves

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