Thinking aloud....everyones going to build muscle at a different rate, dependant on training state, age - hormones, genetics...etc so id assume different people are going to be able to use differing amounts of excess calories in terms of building muscle.
Someone young and newer to lifting would surely be able to generate more muscle from a 500 calorie surplus than someone who is older and has been training for a longer amount of time, or those who had been training for a longer period would be able to gain muscle just as quickly as newbies from the same amount of surplus calories & we know the rate of muscle growth slows with time.
So im not sure we can get a good idea of the best ratio of muscle vs fat that is possible for each of us by generalizing a specific amount of excess calories all the time. Beginners to lifting could probably get more gains in muscle mass from a bigger surplus than someone whod been lifting for a while and again simply individually people could probably use differing amounts.
Apart from that in the end some days your bodys going to need more energy than other days. If you then eat the same amount everyday some of it is going to be stored as fat on the days you dont need it, if you eat lower than that all the time your possibly sacrificing gains on the days your body could use more of those extra calories to grow.
So the most efficient way would surely be to eat slightly less on the days you dont train for example? I know this has been mentioned before & obviously your bodys still growing so you still need a surplus of calories....but you dont need as many as the days your expending a load of calories during and after chucking heavy weights around i wouldnt think.
Why not have it so you eat a 500 calories surplus on training days and a 250 calorie surplus on non training days.
Or a 750 calorie surplus on training days and a 500 calorie surplus on non training days for those who are younger or newer to lifting etc
Any thoughts?