You kind of have to work in percentages with this and do some guesswork!
Start with your basic costs. Printing? Wages? Merch production? Add that up. You have to predict how much of each thing you need, of course, so here are some basics:
Printing: How many are you ordering? Do you want enough to cover the Kickstarter AND have some left over? Are you just selling online afterwards (less) or are you going to cons (more)? It's safe to say you'll need at least 100 backers if you estimate your goal will be in the 3-5k range (this is assuming you're printing BOOKS not ISSUES). Maybe plan for a run of 200 so you have a safe buffer.
Wages: How long do you think it's going to take you to put out this product? Is part of it already done? (I wouldn't suggest kickstarting something that's going to take you more than a year or two, it wears on you) How much do you need to make bills per month? And if you don't already have like 3 years of consistent comic making experience, double the time you think it'll take and add it to your goal. NOTE: For a first Kickstarter I would honestly suggest only doing it when you have 80-100% of the pages already done so there's not much guesswork and your goal isn't inflated by wages.
Merch: Go lower on the estimates for this than the books. Less people will want the bells and whistles. Maybe a third of your estimated 100 backers will want merch, so only plan for like... 30 ish of each item, if not less depending on what it is and what tier you plan on making it. Ideally, I wouldn't do much variety of physical merch unless you already have it made or plan on selling it at cons or something.
Now that you have your basics covered, you need to calculate shipping the rewards. Domestic first class packages in the U.S. run ~$4. International, your average is ~$14. So even if all your backers were domestic, that's $400 for 100 backers. Of course, some of them will get digital tiers and some of them will be international, so you can't calculate exactly. I usually add a $50-100 buffer, so I calculated $500 in mine. This helps, too, in case I end up having to ship 2 packages or something if I offer posters or want to ship the merch separately (some printers will ship the books directly to the backers for you!).
For my first Kickstarter, I also included the ISBN and barcode cost because I wanted to sell them in book stores. Might be something to look into if you're printing a graphic novel!
So after you have all that added together, the Kickstarter takes a flat 5% fee and Stripe (credit card processor) can be anywhere between 2-5%. Best to just round it up to a flat 10% and plan for that.
For my current Kickstarter, my costs came out to:
$6,800 - Wages
$200 - Small issue print run
$500 - Shipping
(I did not include merch because I already have it made)
= $7500
%10 of that would be $750, so I have to add that to my goal:
= $8250
Now, raising the goal means that the 10% fee changes. It's now going to take $825 out, which is about $75 more than what was calculated. I personally pushed my goal up to $8,300 to even it out and decided to bank on the fees not being a full 10%. There's some variance there, and worst case scenario, I'm eating the cost of like $50. Not a big deal to me.
You definitely want to build in a buffer, but don't go crazy with it. You don't want to inflate your goal to where it's double what you actually need or something. You can't change it once you launch!
PROTIP: If at all possible, finish and deliver your product within the calendar year you Kickstart it, because if you don't, your Kickstarter income WILL BE TAXED as profit. This is why fall Kickstarters are generally a bad idea unless you have the book done, because you need to have spent that money by December to not get bit in the ass by this.