The holes should be uniformly distributed across the face. Your panel size with a front plate 19mm thick (and a sealed back of course) would need 228 holes, 10mm dia, leaving the Rockwool inside. This would give you a resonant absorption frequency of 115Hz - and up to 80% efficiency an octave either side if you leave in the Rockwool. This gives a 3% perforation percentage where the minimum is 1%. This can be a grid of 12 x 19 holes
If you use the same drilling pattern with 7mm holes you get 83Hz with 1.44% perforation percentage, you don’t need as much absorption at 83 as at 118 (I calculated it to be 114 from exact 3M width but measured you get 118?) from the plots you posted anyway. It may not be necessary to built the 83Hz as the bandwidth of the 115Hz absorber covers that as long as the absorber cavity is filled.
Link to calculator again:
http://www.mh-audio.nl/Acoustics/HelmholzPanelResonator.html
So all you need to test this is two sheets of timber 600 x 1000, one 19mm thick for the front plate and you could use 12mm for the back seal plate, for a couple of your existing panels. Remember the thickness of the front plate affects the tuning depth of all the ports so if using a different thickness front you need to recalculate the number of holes. I’ve built quite a few of these and they’re very effective.