Just flew a 757-300 (domestic). Slide in front of 7A is about the same distance from the seat as the bulkhead in 8DEF. It protrudes to about the middle of the seat (maybe not quite that much). So you've basically got half a seat of unlimited leg room, more if there's no one in the middle.

8DEF was a bit disappointing because there's no cutout under the bulkhead. Makes things a bit tight on the legs if you like to lean back and stretch legs out.