Your clinic should give you a figure for this, based on your readings (keep a carb/blood sugar diary). I was told that a good starting point is to divide 100 by your total insulin per day, but that is just a guess for a long standing T1 and not necessarily relevant to a LADA who may still be producing their own insulin. It can also vary by time of day and things like exercise eg I am more insulin sensitive when I exercise and less insulin sensitive if I have unusually high blood sugar....
It is a generalisation and I think you can use that as a starting point. As you can see from the responses above, it varies from person to person, temperature, time of day, what your glucose levels are when you inject ( If I get up to 15 then I notice that I need a higher ratio to bring myself down again).
Keep testing and you will intuitively know how much insulin to inject in future