Absolute bros, when is an execution 'yours'

We've all been in that situation where youre fighting tooth and nail with your brothers on absolute difficulty, clawing desperately for any armour you can get. Now when youve been fighting that one majoris and get him to red, obviously hes 'yours'. But what about when youve done 60-80% and suddenly a vanguard comes grappling in, doing his grapple damage and the last few hits to finish him off frantically trying to get his hp back from the kill, or how about a similar situation except an assault has just used his ground pound and one of the majoris is finishable. There are many niche situations but you get the idea, do you tap the execute key or not?