My 1989 TRW catalog shows a 2295 dome piston as 13.1/12.1 cc. No plus or minus.
It shows a 2266 as "flat head" (no cc rating) cause its a flat top with no reliefs.
It shows a 2355 (six pack, four relief flat top) as minus 7cc.
It shows 2399 (my 454 chevy piston, small dome, with an exhaust valve relief) as 13.8 cc. (No plus or minus)
All of these are in a column marked "effective dome volume".