I built a 261 a few years ago,848 head milled only .010, Ross .080 flat top pistons with intake valve relief at zero deck,cam was 212 degrees duration at .050.. and so on.
I measured the 848 chamber at 79 cc's, compressed Felpro head gasket at .052
All added up, this gave an actual static compression ratio of 8.9. With are recurved stock distributor running 12 degrees initial advance ,36 total,vacuum advance limited to about 12 degrees,180 F thermostat the engine ran without detonation on 89-91 octane gas.
This was in a 50 Chevy truck ,3.42 gears and the original truck four speed.
You results
may differ....
All the stock replacement pistons I've seen for 261's have the pistons sitting about .070 down in the hole at TDC.