Ditto- you definitely need the school's calculation policy as schools approach it differently.
We are now consolidating our calculation policy as there were just so many methods for each calculation type add/subtraction/divide etc).
There tend to be two ways of thinking;
1- teach chn a variety of methods so they can choose which works best for them.
2- select one main written method as a school (maybe one in kS1 then a second more developed method in ks2) and stick with it!!
I am totally with number 2. Having moved from a school who used method 2, to a school using method 1, I can confirm that teaching so many different approaches just confuses children!! Of course if a child does not get the chosen method, take them aside in a small group to learn a different approach, but there's no reason to spend a huge amount of time teaching the whole class 4 ways to reach the same outcome. This time can be spent on applying the method to problem solving, which is what the chn are really assessed on.
SO when you get the calculations policy, if there is more than one approach to subtraction, I'd look at it with your child, see which one they find most comfortable, and go with it!