Lath and plaster are normally set on joists at 400 or 450mm centres but they
may differ check in loft or ceiling void to be sure or if no access knock a
hole into ceiling to check,once you have found your first joints measure out
and test drill to mark the rest off the joists. Chaulk line across ceiling

If you really need to come down by 100mm then use a 50x 50mm batten first
screwed through to the ceiling joists with a 100mm screw ,then screw another
to the underside of that. Recommend you use 1800 x 900 x12.5 boards easier
to use for the less experienced.