I am an editor for a trade publication with 40 years of experience in journalism. The company wants me to take a job on the events side, writing content, helping with organization, moderating panels, archiving video. They haven’t called it a promotion but it would clearly be more intense work and involve traveling to conferences every other month. I’d like to accept the offer but I want to negotiate a salary increase after a six-month review. Can I do that upfront or do I wait the six months and in either case, what kind of percentage would be equitable?
40 years experience means you must be close to retirement age and therefore they are not looking at you having a long career there, therefore the amount extra they might be willing to invest in you would not be a great deal if they're 'real' business people.
At that age it's pretty hard to find good paying jobs for many people so I suggest you play it by ear and don't worry too much about the remuneration until it is bought up and focus more on whether the added responsibilities and changes are a good fit for you. Use that for your six months focus and then rethink negotiation amounts. If you're making a big contribution to the businesses bottom line in some way then push for higher pay, if you're treading water or doing things that do not affect their revenue stream directly or can be done by others, then you look at less.
The actual amount is up to you, I don't really care what the market rate is, only what I think I can get from a company. A big change in routine which includes more work or responsibilities I'd push for 25% and negotiate down. If it directly affects their revenue stream and you'd be difficult to replace I've negotiated 100% in the past. At the end of six months you'd be better able to judge your worth.
Lastly a point to ponder. Do you need a raise? Every time you open a negotiation for more money there is an implication that you may leave if you don't get it. You don't know the motivations behind giving you the extra responsibility and travel (sought after by many), they may think it is a reward and take a pay negotiation badly. So judgement call on whether you ask for one or wait until one is offered, but either way I suggest you do so after 6 months.
I don't think anyone can give valid suggestion on percentage. What I would do (given this amount of time) is search for related offers in similar market and apply on them. This should give you understanding how much this kind of job is paying - either by learning about the compensation from interested employers or by probing them with your amounts.
Plus, you might find that your skills are valued more elsewhere.