Drilling concrete
Is it safe to assume that because the underside of the panel is in
close(ish) contact with the base, there's less risk of the concrete
breaking?
"Close-ish" is not good enough. A piece of wood in very firm, uniform
contact with the far side might do the trick. Otherwise, is there any
way you can accurately measure where your breakthrough point will be,
and back drill any oversize hole?
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