Float glass gets you a fairly flat (+/- 0.001"-0.0001") surface, but it's kinda fragile and bendy. You need something sturdy and flat underneath it, which sort

Float glass gets you a fairly flat (+/- 0.001"-0.0001") surface, but it's kinda fragile and bendy. You need something sturdy and flat underneath it, which sort of begs the question. A good surface plate will get you 0.000001"-0.000005"