FAA give guides lines as follows
Separate known aircraft from the aircraft dumping fuel as follows:
IFR aircraft by one of the following:
1. 1,000 feet above it; or in accordance with Vertical Separation Minima, whichever is greater.
2. 2,000 feet below it.
3. 5 miles radar.
4. 5 miles laterally.
Info taken from
here