Please try my very simple little calculator designed to roughly determine your 800m from an inputted 400m and 1500m time.
Performances must be maximal or honest efforts and you must have been in similar shape when running them. It is no good taking your 400m time from when you were injured and your 1500m time 2 years later after consistent training.
Enjoy and please report your findings! I am looking to improve this formula, not declare it as infallible.
1.2 x 400m + 0.216 x 1500m = 800m