Some trainers use the 400NM figure used for 737 NG ETOPS.
Although the figure, is as TP says, for PLANNING purposes only, the ETOPS figure is
wildly inaccurate (but perhaps 'convenient'?) since it is based on a cruise/emergency descent and then a cruise segment. Typical 737 take-off alternate figures are 345/355nm and again as TP says - it SHOULD be specified in your manual as part of your company operating licence documentation, as should the s/e 'over-water' speed for route planning plus the ETOPS figure you quote. Whatever the book says that is what it is, not what some 'trainer' says.