@2old .... I hope that you are reading all this and taking it with a grain of salt also. The thing that I see is that you are
#1... enjoying the feeding of the calves, and doing the chores and being back to "farming" of a sort...
#2.... you are keeping track of what you are putting into them. The biggest expense in starting calves is the milk replacer and the starter/calf grain. So, having $390 in 300 lb calves is actually not too bad. From here on it will get less expensive/ per pound....... by the day or by the animal.
I personally can make some on holsteins by raising them on a nurse cow rather than feeding milk replacer... BUT..... I am feeding the cow grain so she makes more milk so there is still the expense. The last few years I have not been doing it because of an ankle replacement and then both knees replaced. Starting to get back to "doing" with the cattle more...
I think what you are doing is fine... there will not be any "cheap" replacements for some grain... and you do not have to feed enough grain to choke a horse either.
A word to the wise... a holstein or a hol/angus cross will start to "look older" once they hit the 16-18 month age if you have them on pasture and hay like you are planning... they will not bring as good a price if they "look old".... so my suggestion is to sell them while they still have that healthy younger look, and they will get bought by someone that will ship them to a feedlot that specializes in finishing holsteins and dairy crosses. They will go on and "finish" fast if they have had a healthy upbringing... and you will realize a better price too.
If you like doing this with the calves, I would suggest that you start a few more so that you can have some ready to go out on grass when the spring growth spurt hits... In fact, maybe starting calves in the fall, and then they will be in the 3-400 lb range when the good grass growth of April/May gets here... they will better utilize the grass with a little added grain for the protein needs.
And you might find that you are better suited to selling them off at 5-6 wts....
Play with it a little and see where YOUR sweet spot is....
I am glad that your post has triggered so much "discussion"..... and I hope that you can stick around and tell us how they did and when you sold and what you figured you made for your "retirement time" put into them. And yes, I make money on my nurse cows and calves they raise....