I'm a newly hired in a big French high-tech company, as a software developer. It's been almost a year now and everything is fine so far. I've recently finished a project and got moved by my manager to a new team and project, to replace a colleague that will leave soon to another site (to raise his children in a better environment than in a small flat in the suburbs of Paris) and so will no longer be able to work on his current project.
It's been one month that we have been initiating a transition between him and me to teach me what he has done, what will be asked of me, and how to maintain and continue his work. And in two months, I'll be officially the new "him" in my new team, taking all his responsibilities in this new project.
He is a senior software developer with something like 15+ years of experience, though only 3 years in this current firm. I'm pretty sure he is paid at least 50% more than I am (looking at salary grids provided the trade union and considering his position), which I consider is well-deserved considering his experience. My salary is quite fine as it is, but I know some colleagues were given a better starting salary because they have a double diploma (while, in the meantime, I did a gap year working for the present firm, before finishing my studies), which made them lend in a better case of the salary grid.
How should I handle this when it comes to salary negotiations (which by the way will be my first ones)? I know it seems unreasonable to ask for a 50% salary increase, but I can certainly leverage this situation to my advantage.