I find that the nearest standard size fits loosely, a flat sheet can be installed military bunk tight, if that's how you want and are willing to do the work.
Realistically, fitted sheets no longer can be assured to fit "standard" home mattress sizes, because of the great variation in thickness among various models of the same nominal size. Depending on costruction and where it is marketed on economy to super premium prices, a standard full or queen might be anywhere from four to eighteen inches thick. A fitted queen could be sloppy loose on one and not at all fit the other.
Tom Test
Itasca Spirit 29B