My trusty B737-200 Boeing published PTM (Pilot Training Manual)states:
Methodically accomplish the engine failure checklist procedure after the airplane is under control, gear and flaps have been retracted, and a safe altitude has been attained.
Indications of fire, impending engine break-up etc, should be dealt with as soon as possible. The recall items of the appropriate checklist should be methodically completed by or at the command of the Captain
Unquote.
In this PTM, Boeing does not define what is a "Safe" altitude. But it does advise that indications of fire...should be dealt with asap. Clearly, time is absolutely vital with a blazing engine. From the cockpit you may not be able to see how bad is the fire, therefore it would be wise to consider the worst - and act promptly.
A rushed decision at low height could lead to a serious situation if someone pulls the wrong lever due mis-identification. But then look what happened at the Kegworth B737 accident, where mis-identification took place at cruise altitude.
So back to the question of what constitutes a safe altitude for action following a engine fire after V1?
Each company publishes a figure (such as 400 ft agl) primarily for reasons of crew standardization. Could be that 400 feet is too late to pull the fire handle in some instances, and adequate time in other instances. Horses for courses. Which may be why Boeing (for example)do not specify or recommend a specific height, and leave it to the pilot's judgement of the prevailing circumstances, to decide a "safe" height.