You need to get more specifics. Are you going to be flying first class, taking limos, staying in suites, and have a generous expense account? Or are you going to be flying coach, taking a bus, staying in roach motels, with a limited per diem that wont cover crackers from the vending machine?
Also what are you going to be doing? Do you enjoy doing that and are you good at it? Do you enjoy travel and don't mind being away from home more often than not? If this job hits you right in the sweet spot for your ideal job then you do not need to ask for more. For every sacrifice the job will require from you the more money I would ask for.
Another consideration is what are your actual expenses that will increase since you are not home during the week. Do you need to pay for someone to come in and take care of your fish? If you have a spouse and now since you are gone there will be more expenses at home to provide for them or for childcare? It is important to think about everything you will need if you are going to take this job and what the costs will be.
But be careful asking for more than what you are willing to take. You could lose out on a job that would have been yours had you stated your actual salary requirements. But do not cut your throat either. You will not be home to take care of things and there will be expenses you are not counting on. If you take to little because you think the job will be worth it, you may end up broke and unable to take care of your responsibilities.
The TL;DR; is there is no one set formula. This is something you need to think about. If you can get the company to give you the offer you can figure out whether that will cover your needs. If not figure out what you would need if it is more that 25% above the offer, based on my experience, it is highly unlikely that your counter would be accepted. If you do counter being able to explain why you need more might give you some leverage, assuming the needs are reasonable.