Finding of C-alpha C-alpha distance in Restraints file in Tutorial

Dear all
I was going through the tutorial for submission of job in DisVis. But I did not understant the C-alpha C-alpha distance that have been taken as 23. I have gone through the link given and read that paper but I did understand the same.Can any one help me such that how it was taken.
Forgive me as I am new in this field.

The exact distance will depend on the type of cross-linker used.
And there is an addition correction added to that distance to account for the fact we used the CA atom (which is not where the cross-linker is attached).