Thank you very much!
I have managed (using a much smaller and somewhat more detailed simulation) to locate both initial horizons at least up to mass ratios around a factor of 10, which for now is sufficient for my learning purposes. In that regime, a good and quick guess can be made with r(m)=m/2 -0.0215.
I had also made a very silly mistake in thinking that parameter par_b in TwoPunctures sets the position of the m_plus mass exclusively (this is what I incorrectly deduced from the TwoPunctures documentation), and the other one is deduced from center-of-mass being 0. This is of course not true.
In case any future users looked here for answers, for target masses of about ~ m_plus=0.85, m_minus=0.15, I needed to go to 8 refinement levels, where the [xmin,xmax] =[-9,9], dx=0.5.
A further question, if I may - what can I adjust in setting up parameters to minimize the Hamiltonian constraint violation? Best regards Konrad Topolski