It might be more complicated than you realise, depending on where the voter lives.

There are plenty of places where voting Tory stands essentially no chance of resulting in a Tory being elected, especially in this election. So then you need to figure out who could win, and what the effect on prospects for a future Tory government would be for those options - and vote for the one most likely to lead to overall Tory victory, not for the Tory on the ballot.