Are you trying to locate the position of the manhole or read what is embossed on it? A typical manhole is cover is 34 inches (86 cm) diameter. Your GSD of .82cm will produced +/- 105 pixels across the lid. Plenty of coverage for position. You could fly much higher if you are just locating manholes. At GSD .82cm, you will be able to locate rocks a little over 1 inch wide.
No one can answer your question, if we don’t what your requirements are for these manholes. Your drone at 305m (1,000ft) has a GSD of 9cm (3.54 inches). That would produce 9 pixels across a manhole. The bottom line is that the closer your are, the more detail you get. How much do you need?
The attached picture is a manhole at GSD 1.89cm (.74 inches). You can tell it is a manhole, but you can’t read what kind it is.